Make sure you run ANY potential money buyer through www.steamrep.com. Do NOT proceed if you see anything questionable.
Read http://forums.steamrep.com/threads/paypal-cash-trading-best-practices.683/ if you're mainly going to deal with paypal.
Middlemen are always an option.
Going first is okay if the buyer has a combination of 1000+ hrs TF2, $100+ Steam games, a 3+ month old rep thread with at least 3+ pages, or admin on a decently large trading community. Make sure you've cross-referenced the buyer multiple times to catch impersonators.
If you don't feel like doing all that, find yourself a close friend/ teammate who you can trust to sell your stuff safely. If you need clarification on anything, people like Fog or myself would be more than happy to help.