How can anybody get annoyed at a thread, surely you just ignore it?
Meh, it’s a bike forum, not a classified forum. There is one of those, you’re choosing not to use it. These threads are increasingly common. A bit like if there were myriad threads, in the bike forum, about “my dog has x ailment”. I could ignore them, but they’re in the wrong place, and I don’t care, and when they keep coming up it’s annoying.
It’s even more annoying because the answer is the same, every time. It’s worth what someone will pay. There may be a gullible fool out there who’ll pay £3000 for the frame, believing that it’s expensive, so it must be good. I’d pay you £5 gladly.
Therefore, to me, it’s worth £5. To mr hypothetical gullible oaf it’s worth £3000.
If you want to establish it’s market value, put it on the market, don’t randomly ask a group of people who can’t offer any meaningful insight. There are several places to do this. eBay, Gumtree, the Classified forum you’re neglecting to use. It’s not a tough concept to grasp.