As a buyer the first thing I look for is the sellers feedback rating. A seller with a good number of sales and a high positive feedback response (98%+) could generally be considered safe & reliable.
I am personally wary of sellers who don't identify themselves correctly. Sellers with FREE (Hotmail, Yahoo etc) email accounts & no contact telephone number. Be especially careful to make sure you know where the item you are bidding on is located. Some sellers go to extreme lengths to hide the fact that the item may be shipping from overseas. Customs & duty could be a problem for you. I have had a Nokia phone siezed by customs & not returned. Warranty can become a real issue as some local manufacturers (mobile phones in particular) will not support some products & models purchased from outside Australia so check with the local manufacturer first. Some sellers don't clearly show a shipping cost & unfairly insist on profiting, from postage, packaging & handling fees.
If you are selling on Ebay, make your item description is complete and informative with plenty of good photographs. If you have nothing to hide why not include a contact phone number & use your 'real' email address. I guarantee you will get better results.
In brief if you follow these simple tips you shouldn't have a probem at all. Ebay can be a fun experience, however remember Ebay will never email you asking for your personal details or password so beware of scams & change your password regularly.
Good luck & happy bidding (if it feels good - do it)