One way to do that is to research. Here are some ways you can research sellers (on eBay or otherwise)
Take into account that there are a lot of crazy people in this world. Some unreasonable people can often defame a business unreasonably and a grain of salt should be given to some reviews and or complaints of businesses.
If you don't see any problems with a business in your research and you have had a bad experience that has gone unresolved, try posting your experience to various consumer watchdog websites or mentioning it on a myspace blog or something similar. (eBay doesn't allow posting of websites in reviews & guides, each site below has associated websites)
Here are some ways you can do some research on ebay sellers and on retail businesses that you are considering doing business with:
The Better Business Bureau - know that it IS VERY diificult to a bad mark for a business at the Better Business Bureau, often bad companies can con the BBB into not getting the report you make submitted. If you see a bad BBB report be VERY cautious of a retailer.
RIP OFF REPORT - this website dubs itself the "Bad Business Bureau" - in my opinion it's a more accuarate portrayal of bad business.
Specific to eBay sellers is eBayers That Suck - this website is a great resource for all the nearly criminal eBay goings on ... an excellent resource. Know that some of things on this website can be posted by maniacal crazies - still it's sometimes poignant and funny - especially if you've had a bad experience with an eBay seller to see other experiences.
Also specific to eBay, you can search their feedback profile. This may seem obvious, but look at about 2 months worth of positives - most people are afraid to post negative feedback - seeing a lot of negatively worded positive feedback is not a good sign. Also, make sure you look at what kind of feedback a seller leaves for others or whether they keep up with their end of the feedback exchange on a regular basis.
Google the eBay seller or retailer's name - more and more people are reporting about businesses on their blogs and places like MySpace. Google is a great research tool. You can type in the email address of a seller if you know and type in the name of the seller. Google crawls most all websites that report negatively about ebay sellers.