Over the years I have found that a great percentage of the people you will work with on ebay are honest .  It is a shame that some ruin it for others by being dishonest or less than forthright.  I've found it very helpful to do 4 things to avoid falling prey to these people.

1) When buying or selling I go to every prospective buyer's or seller's feedback page and if there are ANY negatives, I look them up.  I read what the negative feedback states.  You can usually tell if it was founded or placed out of anger or some other negative feeling and not, so much the act.  In comparison to other feedback, you can usually tell who is on the up and up.  In some cases, I have even written to other people who have dealt with this person and ask their impressions.

2)  If someone has not complied with or somehow cheated me in the transaction, I write down their name in a little book I keep and I make sure to let them know that I will neither buy from them in the future or sell to them.

3)  I tell my friends to warn them about this person and they write the person to let them know that they are aware of their misdeed. They call their friends who do the same.  Though I do report them, In some cases, EBay cannot do anything about the problem and it's generally good to let the perpetrator know that their feedback is not the only "worthiness" indicator. 

4)  I try to continue to treat every other buyer or seller as an individual because the folks that are inclined to cheat you or take advantage of you will get exactly what they deserve eventually. It all comes back.