I've been using eBay since about 2000. I've bought tons of stuff there... I even had two accounts for most of those years, one for myself and one for business.
In all those years, and all the stuff I bought, I can say that I only really had just a few problems with sellers and those problems got worked out pretty well.
I've also sold a bunch of stuff and that's all worked out pretty well.
Buying stuff on forums... Well... The issue I seem to have had is that sometimes my interpretation of the words "excellent condition" are different from others seem to think it means. Blurry pictures are sometimes done on purpose. Packaging and shipping seems to be an ongoing problem too. I once received stuff for someone else... and it was packed so badly, that I had to repack it before sending it on to the right guy. The guy that got my stuff was a complete dick about the whole thing.... and wouldn't send me my stuff until he got his and I paid him to ship my stuff. I lost out on that one...
Also, you don't stick an original 50+ year old plastic A-pillar trim in an envelope and expect it to arrive in one piece.
Then there are some guys that are pretty honest about what they have.
@C Body Bob comes to mind here.
Selling stuff on forums can be problematic. Usually it's OK, but there seems to be a lot of dreamers out there that want something but don't have the money... or PayPal... or even a checking account.. I do PayPal only now.