For Buying, I only have one recommendation: Esnipe
. You will never get caught in a bidding war, win most of the stuff you bid on, and pay exactly what you want (or less) for it. It doesn't cost me anything for any bids under $25 (because of the promotion running when I signed up), but even without that it only costs ~1% of your bid... well worth it, IMO.
For Selling, just know that there are a LOT of deadbeat bidders out there. Lately I've had to relist things 3-4 times and initiate all kinds of "Non-Paying Bidder" disputes, and it'* getting really old. But it certainly can be rewarding to get a few bucks (or more) out of items that you might have thrown away otherwise.
The more you do it, the better you'll get... buying and selling. Good luck!