Lets get the important part out of the way.

when you teleport to the shops you will be telelported to the admin shop. the first two levels give instructions on the shops and the third floor is the actually admin store. where you can buy things. Note: there is the ability to buy, but more importantly, sell levels to the admins for money. Also if you leave that building and go next door, there is an admin shop that only buys. This will be your primary source of income in the beginning is selling stuff to the admins. Paper is my cash crop as I set up a paper farm to feed my thaumcraft research anyway. But you can sell logs, diamonds, nether stars, lots of stuff check it out.

How to sell to admin is simply go to the sign that has the item you want to sell and click on it. you'll get the message that you sold 1 for $X. there you go you have sold items.

So we have player shops on the server and with chest, redstone, and signs, you can make a shop to buy, or sell, items to other player. Claim a plot with a golden shovel and build the building how you want it. put a sign up and have either "[buy]" or "[sell]" at the top line, this is setting up if you are selling to players, or buying from players. the next two lines you use to describe the item. and the last line is the cost.

after you setup the sign, put the amount (are you selling 1 at a time or a stack) into the chest you setup. then with redstone in your hand punch the chest, then punch the sign. you have now setup a shop. NOTE:if you are wanting to sell 1 item at a time, only have 1 of that item in the chest. if you fill the chest and then link, it will sell everything in the chest for that price, but after you set it up and linked. then you can add more to the chest.