I'm a newbie and wanted to weigh in. Well, I have a complete in box Atari 7800 collection...with the exception of 3 games (Tank Command, Fatal Run, and Summer Games). Summer games is easy to get. Also have 6 Homebrew/Prototype games, including Missing in Action, Sirius, Klax, Super Pac Man, Pac Man Collection, and Junior Pac Man.
So, I guess my hardest game to get was Planet Smashers and I got lucky with Missing in Action...the seller was real cool and I got Klax and Sirius at the same time.
Tank Command still eludes me. I also have a complete Sega CD Collection and can honestly say, my atari collection was fairly easy to get, but overall is more expensive per game. Didnt really think the Atari 7800 collection would be that expensive...I'm probably overpaying, lol.
When I complete the collection, I'll wait a few years and try and sell the whole thing with the Sega CD collection for 1 gazillion dollars...almost ready to retire, lol.