I look online for good deals on saddle pads and turnout blankets because it seems to me that they are always overpriced in stores!
And you could probably never go wrong with grooming supplies like shampoos, mane & tail conditioners, etc... all that stuff.. I just wouldn't waste my time selling things like brushes/hoof picks just because they're already super cheap and I'd imagine people would rather just go down to the store to grab what they need rather than order it online and wait for it.
Also... side thought.... I don't really know what your style is, but I'm always looking for PRETTY (but good quality) equipment for my horse! Example: I bought lilly pink bell boots and pink & black leopard print wraps yesterday. I'm buying her a cute plaid pink turn-out blanket soon, and a black and pink saddle pad to go with her black and pink saddle. That stuff is surprisingly hard to come by (good quality, at least), probably because the majority of horse people don't really care about that kind of "look".