At first I had the game setup so that when you wanted to sell an item, you simply opened your inventory, clicked the dollar sign next to the item, received a set amount of money and that was it. It was a very unengaged process, so I made some changes.
If you you've watched the game's progress before, you'll know it's a menu heavy, management game and not a actiony adventure game.
Various stores will buy your items. They seek one type of item at a time, and pay a percentage of the item's value. The items they seek and the prices they offer will change once their offer runs out and gets refreshed.
They're similar to the above, but contracts require multiple items and have to completed within a time limit. If accept a contract but don't meet the requirements, you'll receive a fine and also lose any items you've put into the contract. Contracts almost always pay better than distributors.
Once you've completed a contract or sold an item to a distributor, the shippers will show up at midnight to collect the items and pay you. Your finances might get dicey if you need money right now but have to wait until midnight for your payments to arrive.