for an honest transaction, with good people, you wont find any better than here, all the facebook groups are full of scam artists who dont reveal the entire truth about a bike short comings, tings are passed off as nos when they are in fact polished turds.
ebay is a mine field, ive seen bikes for sale on there that dont even belong to the person selling them, same as gumtree, stay away from there as well.
There are a few forums still around, most of the guys on BMX are legit, thats if you can get them to ship outside the lower 48. theres a couple of burner forums that are quiet now but still around. Other than that theres nothing