If you go just south of spawn and turn left at the sign to the Merchant District, there is one shop (thrown together... sue me)
It has Gadget-type tools, Music stuff, and redstone...
You can buy and sell, based on the amount you have/want, and the prices on the signs.
Check your balance with: /balance
Hold something in your hand and type /worth to see what it is worth (NOTE: This is going to be in flux for awhile, and the /worth $ may be different than a store has.
Tomorrow I will FO SHO be getting the plots lined out and priced.
PEACE.