and remember to consider all expenses involved in selling the item. cost to get it (gas, shipping to you, etc), ebay fees, paypal fees, packaging expenses, postage, time and labor for photographing, listing, packaging, etc.
i agree with sell what you know (or at least be a very smart and fast learner about new product). If you dont know the product, you wont know if youre getting a good price when you buy, wont know if youre getting ripped off or not, etc.
For example - I wont buy gemstones from companies I'm not familiar with because I dont know gemstones well enough to spot a fake with naked eye. Silver I can tell at a glance if plated, etc. Turquoise i can tell at a glance if real or fake. Same with mother-of-pearl, coral, and several other semi-precious stones. but several gemstones (sapphire, topaz, ruby, etc) I dont know well enough to spot the fake with naked eye.
On an aside, regarding the stones I can spot, i've seen several fakes being sold as real on ebay, dunno if the sellers were intentionally doing it, just didnt know and assumed was real, or if they got ripped off and didnt know it. and such things arent limited to just ebay; i've spotted fake stones being sold as real at major retailers including wal-mart!
Dooney Burke purses, i can usually immediately spot the fake (thanks to training by a friend of mine). Whiting & Davis I can spot the fake bag. But I wouldnt attempt to buy other designer bags cause dont know them well enough.
wholesalers - some are real wholesalers, many just claim to be wholesalers. easiest way to sort out is to compare price they offer on items to prices items sell for online.
couple of other tips - check ebay to see which markets are flooded with product. jewelry is overloaded so hard to sell and hard to get good price. same with several other categories. look around you for sources for product. in most cities there's several manufacturers of stuff right at your back door and several places to get stuff on the cheap. Sometimes that's salvation army, sometimes that's yard sales, sometimes it's dumpsters!
ALWAYS think outside the box. and sometimes you can even find bargains on ebay that you can turn right around and sell for a profit
(tip: buy with one id and sell with different selling id.)