Harriet Walker: Once we used to donate stuff. Now we just want to flog it

You can tell when people start spring cleaning in my neck of the woods, because every Monday morning there are black binbags sitting outside Age Concern, ripped open as if by a pack of velociraptors, their contents thoroughly rummaged through and scattered to the winds.

This, then, is modern charity.

Further afield, Viscount Cowdray had a bit of a sort-out this week too. He has sized up the grandiose trappings of his family seat and is holding an enormous public sale. His Gainsborough, not seen for 50 years, will be auctioned off at Christie's to the tune of £6m, while there are carriage clocks for a more manageable £100. I don't suggest that M'Lud take his personal fortune down to Oxfam – far from it. In these straitened times, we're all obliged to find profit wherever it lurks, whether Cash in the Attic or a Rubens in the ante-chapel.

But not all of us are viscounts. Not all of us have things that other people actually want; that's why we give them to charity shops, in the hope that they might, just might, turn out to be useful to someone else. Or that's why we used to, at least.

Last year eBay reported revenues of some $2.2bn; its biggest annual boomtime is March and April, when people begin to overhaul their affairs while the sun shines. In some ways, there is nothing wrong with uploading your old toot and seeing what you can get for it. In fact, it makes more sense than throwing an unwanted fridge on top of the white goods mountain that threatens to ingest our planet any day now; it keeps a certain fluidity of movement alive in the second-hand goods market in an age where people are prone to save themselves the hassle and simply buy brand new.

But there are limits to what it is seemly to flog online. As well as cars, furniture and ceramics, you can also bid £3 for someone's old M&S cardigan, or a pair of designer shoes so scuffed and leaky soled they look even older than Viscount Cowdray's Gainsborough. I know, I know: one man's tat is another man's treasure, but there has to be a limit to our craven need to extract – literally – money for old rope.

It seems to me that charity shops fulfil a very different purpose from eBay, and it's one we haven't quite got our heads round yet: they're not for things we don't need, so much as the things we don't need money for.

With the ease of the modern age goes some of the thrift and hardship of yesteryear. We don't make do, let alone mend; we chuck out and charge it to our cards. But all this material turnover has made us even more parsimonious and much less philanthropic. The vest we are planning to sell on eBay for £1.50 (plus postage and packing) could just as well go into the binbag and along to the charity shop.

Restrictive undergarments aside, the Victorians have a lot to teach us. They knew the value of a thing, because of the efforts that went into its creation and the lengths they had to go to get it – but they also recognised its human, rather than its monetary, worth. The cult of philanthropy meant that things were handed down for practical use by others, not hawked about for mean-spirited profit.

I am an insatiable hoarder, but just one rifle through the ever-expanding bag on the landing convinced me that putting the contents on to eBay would be a shameful shunning of social responsibility. You don't need to be Viscount Cowdray these days to be higher up the food chain than someone else.

So instead I'll be dragging it along the road this bank holiday to Age Concern – and before it closes for the day, so that drunken vultures and light-fingered opportunists don't get the chance to have a pick through it first.