Nstars, exp, and most mining materials are generally the most popular. Usually shops use trade signs to buy Nstars and Exp, then they resell for a higher price. As for ores and other blocks, this method can work, but its much slower and your better off gathering materials on your own.

As for getting it, click this link --- http://tinyurl.com/98vbxvf

 

I normally garter my own supply/make.For nstar and exp buy and sell for a little profit at the shop.
hope this helps

 

Many of the best sellers include things such as exp, Nstars, building materials, and many different types of minerals. Try spending an hour or so to browse around the market to figure out what the other shops seem to be selling and their prices. From there, you can work your way up to making a list of stuff to sell and etc.
To getting this stuff you have 2 methods:
1. Mining/gathering everything yourself which allows you to keep 100% of the profits
2. Buying for cheaper and selling for higher allows you to make profit, but unless many people buy and sell, it's much slower. (Large top floor spawn shops use this method a lot)