Thanx to WarBoundification for this workaround!
This solution is most likely the easiest and fastest way do gain XP, but it has two conditions; The challenge "Merchant of death" must be unobtained. (sell 1200 items / 20,000XP), and you need some cash.
Say you start from scratch (0 sold items), you need $287,760 to do this trick, so you have preferably already completed the game before even reading this.
What is going to happen is this; you will gain 20,000XP for every save/load.
Here's what to do:
-Travel to Fyrestone, and go the Zed's vending machine (cheaper)
-Buy as many Light Healing Kits you can fit in your Backpack
-Sell them just as fast
-Rinse and repeat 'till you reach the amount of 1,199 sold items in your Challenge Log.
-SAVE
It is most important that you save at 1,199 sold items, NOT more! If you save at 1,200, you will have lost this golden opportunity. So check your log!
Now the good stuff:
-Load
-Return to Zed's vending machine
-Sell one item of your choice, and BUY IT BACK (Use the "Buyback" option)
-Exit vending machine
Now you will receive 20,000XP for completing the challenge "Merchant of Death", but since you bought the last item back, it will be deducted from the counter, and still show 1,199!
Save yet again to register the newly added 20,000XP, and load her up again.
There is no limit for how many times you can repeat this.