Guide for fast on-demand XP in Borderlands 1 (Merchant of Death glitch)

Borderlands 1 is punishing when you're underleveled and you need to be diligent with sidequests to avoid being underleveled at all times. Abusing a glitch with the Merchant of Death challenge allows you to get 20k XP on demand which is a significant chunk until the late game and can be used throughout the playthrough if done properly.

Steps:

1. Have at least 150k (Guide for early money) and the Merchant of Death challenge not yet completed (Sell 1200 items)
2. Empty your inventory as much as possible (ideally all minus your equipped slots)
3. Go to a health vending machine
4. Purchase as many 'Minor Healing Kits' as you can
5. Sell the kits (Do not buy them back)
6. Repeat until 1199 items have been sold (Check the challenge progress in menu under Cassette icon -> Challenges/Medal tab)
7. Purchase 1 more kit
8. Sell the kit
9. Buy it back (Otherwise you won't be able to repeat the challenge)
10. Pause menu -> Exit (quitting to main menu while saving progress)
11. Re-enter game and repeat steps 7-10

[Note: when you load into the game you need to wait at least 30 seconds before you save quit for the game to force a save. Otherwise, your progress won't be saved if you quit out again too early]

When you buy back the last item needed to complete the challenge, the challenge itself goes back to being at 1199/1200 after you have already gotten the 20k XP which is what allows you to do this over and over again each time you save quit and re-enter.

Bonus: How not to wreck your hands doing this

If you're on PC, you can avoid spamming the enter key 2400 times by installing AutoHotkey and using this script:

#NoEnv  ; Recommended for performance and compatibility with future AutoHotkey releases.
; #Warn  ; Enable warnings to assist with detecting common errors.
SendMode Input  ; Recommended for new scripts due to its superior speed and reliability.
SetWorkingDir %A_ScriptDir%  ; Ensures a consistent starting directory.

Shift::
Send {enter 15} 
return

This makes your Shift key do 15 instant Enter presses (you can adjust that number to match your free inventory space). Doing this can let you get to 1199 much faster than manual and takes less effort.