Probably if you are looking at profit margin over 20% it will have to be electronic items like popular mobile phones. But then again, you'll be probably buying small bulks however others will pick larger bulks because they are assured they can run down the stock at anytime at a profit, so they will get a good discount.
If you are looking at a higher profit margin, it will have to be cheap products ranging from $1-$5 in niche market. You have to do manual market research to get to find out products which will do well, they could be simple yet latest gadgets like computer mouse.
Electronics are a bit of hastle when you buy in bulk. Let's look at clothing, clothing could actually do well, but in recent times there are a lot of fakes being sold. If you are selling authentic items ( which i recommend ) then you should make sure you market your product as being authentic and not being afraid to take real closeup pictures of the clothes and tags, this will help you get the sales you desire.
Honestly, as someone mentioned above, no one will really answer your question straight-forward. You probably should start off in a JV with an established eBay seller, could be very fruitful.
The business environment in recent times is constantly changing, what is a NICHE today is NOT A NICHE tomorrow. Competition sneaks in very quickly, you are making a 100% markup and then overnight someone starts a listing which kills your 100% markup to maybe 5%

I dont mean to discourage you, but no one will really reveal their niches so easily. You have work hard for it and dig it out through concrete!