Creating $ value for items

I would like to create a game which involves being able to buy stuff as well as sell. For example lets say there is an item the player can buy, how would I set a price for that? Also I would like to have a resell option, which they could resell for a different price, and have multiple options to sell at different prices for each place. Say there is an item that they could buy for $5, then resell it online for $20, or they could bring to a pawn shop or something and only be able to get $15 for it there. How would I go about doing this? Any good tutorials out there I may have missed?