marketing isn't too difficult, it just takes patience. If you try to rush things you won't get very far.
You generally want to go off of market price, not shop price.
Depending on how you want to sell, you'll want to buy at a certain percent off. If you plan on solely looking for really good deals and then making your own thread to sell, you'll want anywhere from %30-%50 off of the item. If you want to take it slow and make the most profit possible by selling on the market, buy at at least %15 percent off market price, and put your item on the market for 1 tc off the current price. You'll need to check it every now and then and update it if someone does the same is you, but as long as you have the lowest market price, if anyone needs that kind of item, you'll be the first person on the list.
You also need to be weary of the popularity of an item. If you see a 3D item going for like %40 of its market price, its not very popular and will be hard to sell.
I'd recommend sticking to forces/relaxes/bloods/textures. Buy mid-tier forces/relaxes. Pure/void are hard to profit on. Aim for Hunter, QS, Cobra, things like that. You'll eventually get an idea of what is desired and what isn't.
