Unfortunately right now we are in the middle (my this is a long lasting middle) of getting things organised. We are working very hard on getting lists up that will easily allow people to know what is available and what is needed. My best advice right now would be to post a list of what you have available.
The easy answer to your question about what is wanted is really stuff that is video game related (we have been having a problem recently with a certain member trying to confuse new members on this point). That means console video games (PSOne, Atari, SNES, etc) and also computer games (PC, Amiga, Atari-my they got around, Windows, etc). Recently we have started accepting comic books based on video games too.
What we don't accept here (mainly because of storage constraints) are manuals. That is why I went looking for other sites that have similar interests as Retromags and was extremely happy to come across the Replacementdocs web site.
A key point to make sure to watch though, is that anything you want to offer for download here on Retromags needs to be printed before January 2000, December 1999 and earlier is good.