At Sotheby's they can fetch pounds 2m. Chris Martin will sell you a similar ancient artefact for under pounds 100. By John Windsor
Sumerian stone beads from 3000BC, the dawn of civilisation, cascaded on to the office desk and disappeared into the pile of the carpet. Chris Martin, Britain's biggest mail-order antiquities dealer, had accidentally broken the string of a bead necklace as he prised it off a display card. There was no rush to pick them up. One of his two women helpers, busy re-stringing boxfuls of the ancient black, white and red beads, said: "We'll see to those tomorrow". I paid Mr Martin pounds 7, trade price, for a 19in necklace of Sumerian beads. A bargain. His mail-order price is pounds 14.95.

Antiquities are plentiful and cheap. The lament of antiquities dealers, auctioneers and fair organisers is that not many people know that.

Next week is antiquities week in London. Tomorrow there is a fair organised by the Antiquities Dealers Association and, during the week, there are four auctions at Christie's, Sotheby's, Bonhams and Phillips. It is a chance to discover how cheaply you can buy millennia-old stone or classical pottery.

Some of Mr Martin's stock for beginners is stored in plastic crates in his office in north London, bearing labels such as "Pots - Roman - small". A plain Roman domestic pottery bowl costs pounds 20 by mail order. So does a Roman terracotta oil lamp from Palestine of the kind he supplies by the thousand to promoters of mail-order offers. One promoter's newspaper advertisement says: "Own something from the 4th century AD: genuine Roman oil lamp, only pounds 39.95".

Such lamps change hands in the trade for a mere pounds 10 (those with decorations of animals or erotic scenes fetch many times more). The packaging can cost more than the lamp. Mr Martin showed me a cardboard box containing a polished slate display stand that had cost pounds 11 to produce. Roman oil lamps are sold through TV shopping channels here and in the States.

Britain, thanks to its history of classical education, has more antiquities in circulation than any other country - some five million according to one collector, with another 100,000 or so small artefacts such as pots, statuettes, pins and brooches being unearthed every year by metal detectors and developers. No wonder the prices are low.

Mr Martin retails Palestinian and Phoenician cloak pins from 1500BC to 1000BC for pounds 15 and Persian dress brooches (8th-5th centuries BC) for pounds 11. Roman glass? He charges pounds 50 for a simple bottle. For pounds 32.50: a perspex stand with a Greek or Roman egg-shaped lead slingshot, a Greek bronze arrowhead (8th-3rd century BC) and a Roman iron arrowhead (3rd-2nd century BC). Dagger and spear blades? He retails by the dozen Phoenician and Hittite specimens (2400-800BC) for pounds 30-pounds 40 each to multinational companies who present them to distinguished guests as paper knives.

But the apparent surfeit of antiquities is not what it seems. Supplies from abroad to British dealers have declined dramatically in the past six months because British Customs, allied with Scotland Yard's Art and Antiques squad, have been seizing consignments.

Antiquities have been in free circulation for millennia. Britain, as a non-signatory to the Unesco convention prohibiting the transportation of unregistered antiquities, has hitherto allowed free entry. But times have changed. The dealers, auctioneers and collector I visited were outraged and jumpy - who would the police pounce on next? For every Art and Antiques squad horror story of hieroglyphic stone slabs ripped with chainsaws from Egyptian pharaohs' tombs, they had one about impoverished Near Eastern peasant farmers forbidden to dig up crockery from ancient graveyards the size of football pitches.

Some dealers reckon that Scotland Yard's ear has been bent by a coterie of archaeologists who believe that only museums should own antiquities and that collectors are no better than looters.

Agreement could take years. Meanwhile, would-be collectors have what could be their last chance to pick up, cheaply, the breath-takingly beautiful artefacts of ancient civilisations. When it comes to pottery, most collectors still tend to buy for archaeological rather than artistic reasons, so the most unattainable prices are for pottery from classical Greece (6th to 4th century BC) and Rome (1st century BC to 5th century AD). Which means there are bargains in Greek-influenced pottery made outside those regions. As Michael Harrison, antiquities collector and consultant, puts it: "If you have an eye for art, you can buy exceedingly good things cheaply." But be cautious, especially on holiday. Wares from Pompei's souvenir shop are particularly beguiling, but there are as many fake antiquities in circulation as there are real ones. Go to reputable dealers, who will supply a certificate of authenticity.

If it is sheer antiquity rather than aesthetics you are after, start with Mesopotamia and Egypt, around 4000-3000BC. Coarse red polished Egyptian domestic bowls of the period, with characteristic black rims or tops are sold by Mr Martin for pounds 500; Cypriot black tops of 3000BC for pounds 120; and Palestinian pots, of 3000BC or earlier, for pounds 100.

For classic Greek shapes, cheaper than original Attic products, go for pots from southern Italy made under the tutelage of Greek colonists in the 4th century BC, a century after the flowering of Attic pottery. A fine 41/2in kantharos (cup-shaped, with loop handles) might cost pounds 300 from Mr Martin; an 8in bell krater with decorative figures, pounds 500; a 91/2in with figures pounds 1,200; and a decorated 11in at pounds 2,000 - his highest price for an antiquity.

Mr Martin put a pot similar to his pounds 1,200 specimen in Bonhams' April sale where it was estimated pounds 900-pounds 1,000 and fetched pounds 1,430. It goes to show that antiquities are not always cheapest at auction. London's West End dealers tend to charge more than auction prices and smaller dealers like Mr Martin less.

The same sale showed undecorated wares are still out of favour with established collectors, especially if made in southern Italy. A 12in- high 4th-century BC black-glazed krater with minor repairs, estimated pounds 700-pounds 900, failed to sell.

My own favourite is the expertly potted and charmingly decorated small 4th-century BC Hellenistic kantharoi from the Daunian region of Italy. They are so cheap (under pounds 100 to pounds 200) and in such slight demand that they are seldom seen at auction. Also largely unwelcome to auctioneers is Etruscan ware of 6th-7th century BC, and the chunky pots of the third millennium BC Minoan civilisation and second millennium BC Mycenaean. If you can cultivate a taste for these, your pocket will be less stretched.

I visited London's top antiquities dealer, James Ede. His father Charles's guide to antiquities collecting is still the most accessible (pounds 19.50 plus pounds 1.50 p&p from Mr Ede). He has a decorated 440BC Greek stemmed kylix - wide shallow wine vessel with handles - for pounds 18,000. It is decorated with what I took to be "one of those homosexual scenes". "No," he said, rather sternly, "it is two youths talking."

Antiquities people are passionate about ancient cultures, Mr Ede said: "We forget what sophisticated societies existed around the Mediterranean from 3000BC. Look what the Egyptians were building in 2800BC. It beggars the mind. Now it rains in Karnak because of the Aswan dam and the Valley of the Kings is falling apart. What a terrible cock-up."

ADA Antiquities Fair, Inter-Continental Britannia Hotel, tomorrow (10am- 4.30pm), entry pounds 2.50 (0181-979 1585). ADA members directory: (0171-930 1864). Auctions: Monday, Phillips (2pm), Tuesday, Bonhams (11am), Wednesday, Christie's (10.30am), Thursday, Sotheby's (10.30am). Christopher Martin ( 0181-882 1509). Charles Ede Ltd, (0171-493 4944).