Check seller feedback
Before I purchase anything on eBay, I always check seller feedback to make sure they have good reputation. Checking feedback is easy. You just click on their feedback score and it will take you to their eBay Feedback Profile. On that page, you can see what people have to say about their past transactions with this seller.
If I see any negative comments, I like to read what people had to say about them To filter out positive feedback scores and only see negative, you can click on the number next to Negative and that will display only negative comments. After reading those comments, I make a decision to continue further research or move on to another seller.
Check recent eBay selling history
Sometimes, I want to know what the seller sold in the past, especially if I'm interested in an expensive item. eBay allows you to see recent selling history for a given seller. There are multiple way to get to that screen. One of them is to go to advanced search page and choose items by seller:

You would just enter seller User Id, check "Show completed listings only" checkbox and hit Search. This will show you all eBay auctions and Buy It Now items that this seller sold in the past 15 days. If the seller was selling some cheap items in the past and suddenly listed some expensive item, that usually raises a red flag for me. I would think twice or ask seller some questions before committing to buy.
Conclusion
Before purchasing anything on eBay, do some research on the seller to make sure they're reputable. First thing you should do is check their feedback score. If feedback looks good you might also want to see what they sold in the past.
If you liked this post, check out my other blog posts for more eBay buying tips. I also invite you to try the eBay buyer software that I created. It will help you find good deals and save you money.
Robert