Its pretty simple really...
Buy shit people want (whether you personally like the things or not)
Buy shit that needs cleaning, not repairing
Only buy them if they're at the right price (don't be worried about walking away from 'the deal of a lifetime,' because one comes along every month)
Be nice to people... Endearing yourself to the person you're buying from/selling to makes them feel more comfortable about the deal.
That's about all there is to it...