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!

jbidwatcher will snipe auctions too, i suggested it because its free