The best way to know if a seller is trustworthy or reliable is by looking at previous transactions. After each transaction a buyer is allowed to rate the seller by means of feedback. Feedback consists of a rating system, users can rate their experience Positive, Negative, or Neutral.

Why Feedback is important?
eBay users are separated into two distinct categories, sellers and buyers. Feedback protects both the buyer and seller by letting you determine a user's reliability based on past performance. Why is this so important? If a seller is prone to shipping items late, not packaging properly or doesn't respond to emails these would be red flags telling you what to expect if you were to deal with them. For the most part feedback is pretty accurate and can paint a pretty good picture of a sellers past.

Feedback also let's a seller know that you are trustworthy and good to deal with. Do you pay right away? are your checks good? if there were any problem did you give the seller a chance to make you happy? etc.

The feedback system is easy to understand. You receive:
+1 point to your feedback score for each positive comment and rating left for you.
0 points to your feedback score for each neutral comment and rating left for you.
-1 point to your feedback score for each negative comment and rating left for you.