This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
There are various web sites available now that list second hand adverts without charge. If you are wanting to sell some equipment it may well be worth your time trying them. They are:-
The Acorn User Magazine website normally carries free ads, but you can also advertise for free in the magazine by emailing your advert to; firstname.lastname@example.org
There is also a large Acorn/RISC OS section on the UK Auction site eBay (www.ebay.co.uk)
You can also advertise items for sale on the relevant Acorn Usenet newsgroup. I would advise advertising hardware and peripherals on comp.sys.acorn.hardware and any software items on comp.sys.acorn.misc Please respect Usenet 'netiguette' by refraining from posting your advert to multiple newsgroups (known as cross-posting).