I'm in the "new and shiny" league myself, so most of the tinplate I have is of recent vintage, and some of it was sold to one individual prior to my move late last year. I still have a good amount of O tinplate though (mostly LCT and MTH) that I plan to sell in the near future while holding onto only a very few favorites. Even with a dedicated and decent-size train room and lounge area, I have run out of layout, display, and storage space.
My earlier buying pretty much all was done at a local train store, at York, and via e-mail or online orders with several dealers around the country. I avoid eBay and auction sites, although I was an early eBay seller and buyer. Haven't used them for years now...too much hassle and too many fees. I nowdo most of my selling via our forum or personal contacts.