"Self contained economy" and "Merchant of Death" challenges hint
Once you have about $500,000, go to any medical vending machine and purchase as many light healing kits as you can hold. Note: This becomes easier, depending on how many Backpack SDU's you have obtained. Sell all the light healing kits back. You will lose money via this transaction, but it is an easy way to earn a combined 55,000 experience, if you have not completed any of the challenges for purchasing or selling items. Even if you have, the 20,000 experience point reward for the final challenges of selling and buying items is worth the money alone.