BY buying i assume you mean tips to follow before picking a seller. Being someone who sells a lot on ebay these are the rules i follow before buying.
1 - Be careful of NWOT, if you want to truly ensure something is new, buy it new with tags, because just because it is NWOT just means you cannot prove it was worn.
2 - Be careful of any dvd's, video games, blu rays, that are new but opened... Unless it is sealed, assume it was used at least once.
3 - Only buy off people who have at east 10 feedback... Feedback is easy to come by, and if someone has zero, either they are new and might need more experience or help, or it could be someone stealing your money *(not always, but i don't take the chance)
4 - If selling, do not EVER expect to get retail for your items, unless they are new release electronics, beyond that, your items usually sell for about 50% off..
5 - Be realistic and pay attention to shipping, its usually in fine print in listings, and you don't want to accidentally buy a dvd for 25 dollars shipping.