The best place to sell your stuff is Mirv. He buys all your stuff back for at least half of what you paid for it. The cheapest place to buy boosters are Mirv and the bots near Thalia and Serverton 3000. Mirv sells the cheapest stuff and buys them back for the highest price you can get. He is the best place to buy and sell stuff so remember to always go to him to sell something.