The most important factor to a sucessful and pleasant Ebay purchasing experience is RESEARCH.
Research into 1) what you want to buy, 2) how much the item generally costs, 3) who and where you will buy it from and 4) what the freight charges will be, will all ensure that you end up with the desired item at a price you can live with - if you achieve this as a minimum you are doing OK - and you may even get a bargin - the hope of which is what keeps all Ebay-ers going!
1) What you want....READ the item description well. Is it what you want precisely? There is nothing more disheartening to discover that you have purchased the wrong thing and it's all your fault because you didn't read the description. Most sellers of repute tell it like it is...ie the item is new/old/used/scratched/has no box/is a demo etc etc. If you are in doubt ask a questions of the seller...again most sellers of repute have no problems answering questions.
2) Cost of the item......everybody loves a bargin - no body likes to know they paid too much. If you care about the money, find out what the item costs retail, or from other sellers on Ebay...DON'T just bid on the first item you see. A bit of research may lead you to consider other models/features etc on the item that you didn't even know existed yet on that product. Remember to check freight charges on the item you are looking at also...it affectes the final price you pay!
3)Who to buy from?? On Ebay there are fly by night crooks - thankfully very few. Feedback is the key. Deal with people who have been selling on Ebay for a while, have sold many items and received positive feedback - these people want your buying experience to be a positive one. Research their feedback profile.
4)Freight charges; this sort of ties in with the "where" the seller is issue...If an item has to come from another country, then it's going to cost more to get it to you. Consider local sellers in your country unless the price is just too good to ignore. Some sellers seem to obtain revenue from the freight on their items, rather from the item itself - there is nothing wrong with this as most have their freight fees outlined in the item description...again..READ IT ALL.
Like most things, the more you practise, the better you become. Remember to always be polite when there is a problem (it costs nothing and can help a great deal) and always leave feedback