I've sold stuff on gumtree and facebook marketplace.
but as your after specific models you'll probably still be playing the waiting game.

I'd just use local ads, whatever the UK equivalent of Craigslist or Kijiji is. Or see if there are any electronics recyclers or local vintage computer clubs you can reach out to try to find something locally.

The other option is to just YOLO it and get something from Ebay. Whenever I have gone after specific make/models, invariably I've had to import them from the U.S.

Car boot sales, local auction houses when they do random household item sales or business equipment. General town market flea markets. Backstreet second hand stores. Get a "free sample business card" run done of cards with "Wanted, Compaq computers and parts of any age, phone .... ... .. " then leave them up on pinboards, in newsagent windows etc all round. When you do that, take everything you can, use it for practice, fix up and pass on if it's not your thing, might even increase your war chest for buying pristine examples.

Anyway, it's all a numbers game, can't try one or two things a couple of times a year and go "Can't find anything" because at that rate you'll take 100 years to get lucky. It's like there's two jokers shuffled into the deck, gotta keep drawing cards to get one, just coz you don't get one 20 times in a row doesn't mean you're not getting one. Humans evolved as persistance hunters, so ... persist.

I'm in Canada now, and recently my highest hit rates have been at what would in UK be called "Church rummage sales" or Church or related groups jumble sales. 1 in 3 has had something interesting. Got an IBM PC330 486 from one last year.

Ham flea markets often have older computer stuff here, in small proportion to the mountains of radio and associated gear of course, there's gonna be one next Saturday in Nottinghamshire, https://www.nationalhamfest.org.uk/flea-market/ fiver for general admission, more maps on the main event page if you click the logo on top left.

edit: I guess those are actually my highest hit rate, mostly seem to buy parts only though, church sales are highest hit rate for general non specialist event.

I have bought pc stuff from local charity shops ie bhf oxfam etc quality can vary from really poor to new in box stuff . And do try local car boots or charity jumble sales.

The only places I seem to have any luck buying hardware is Facebook marketplace, Gumtree and eBay. And it's not often that something pre-2000 comes up on FB and Gumtree to be honest.

Charity shops and CEX are good for cheap games through.

BitWrangler wrote on 2022-10-12, 17:19:

Get a "free sample business card" run done of cards with "Wanted, Compaq computers and parts of any age, phone .... ... .. " then leave them up on pinboards, in newsagent windows etc all round.

I've thought about this for some time and will probably try it.
But I'll write something more generic like: "Looking for old computers, ca 1980-2000"

BitWrangler wrote on 2022-10-12, 17:19:

When you do that, take everything you can, use it for practice, fix up and pass on if it's not your thing, might even increase your war chest for buying pristine examples.

Yeah. Generally if it's way below the usual Ebay price, just take the whole machine/lot. Even if you only really want one or two pieces. Asking for these might make the seller suspicious and check Ebay for them. Suddenly 50 bucks for everything becomes 80 bucks each for your two items. Just take it all.
Test it, fix it, clean it. And then off to Ebay with it. With a more realistic price expectation than the competition. Or trade with other collectors.

I wonder how successful it would be to try a "New(er) for Old!" program... "I take your old pre-2001 computer and trade you for a refurbished machine capable of modern web browsing and office work."

Your best asset in developed countries is skill. Look at the price of a faucet vs plumber visit. If anything offer help migrating data and setting stuff up for old computers.