I must admit there have been sellers I've dealt with who haven't been good to deal with and it's frustrating for me as often they have items I'd love to bid/buy but I'm wary of using them.
The guideline I follow in this instance is to limit how much I'm prepared to spend with them in case it all goes pear-shaped, which in turn limits how much potentially I could lose on the transaction.
eBay and Paypal can't be relied upon to refund your money either, especially if you've been left waiting and waiting for items to arrive from overseas and the time limit has expired.
I usually let sellers know when I've received their items by email and if all is well then feedback is left for them soon afterwards. If there is anything which needs resolving then feedback is withheld till it's sorted one way or another.
Most sellers who I contact like this are genuinely delighted and if you work to the philosophy of 'treating others how you'd like to be treated yourself' then you can't go far wrong.

I appreciate it's difficult to keep a track of auctions and people if you're selling a great deal of stuff all the time, but surely there are programs which can do this for you?
The way I manage it is to have a folder for each of the sellers I've bought from marked either 'pending', 'paid' or 'recd' against every item, so I can see who needs paying, who has been paid and who I can leave feedback for once I've received my item. It might sound a laborious way of doing things, but as I'm using Outlook Express as my main email program then it's the method which works for me.
