When I was playing this game for 7 time, I found a way to sell ammo. This can be done only once for each weapon, so you may wanna be patient in order to get as much money as possible.

So to make this trick, you will have to be very patient and skilled enough to survive, especially if you play on hard difficulty. Hard difficulty is recommended, as you get the most experience there. At first, get as much experience as possible in order to have skill points. You need to survive till level 6, if you play on hard difficulty so you will have 10 skill points. When you have 10 skill points, add them all to strength. When it's done buy the weapon you need. When you have bought the weapon, you will see that you also received full ammo stock for it. Reset all skill points you added to strength. When you will be reseting them one by one, you will see that you are getting money for it as if you sell ammo for the weapon. IMPORTANT NOTE: if you find a weapon for the first time, do not pick it up. if you do, the trick will be wasted for this weapon. Playing through without weapons can be impossible, so you may want to get yourself a shotgun and a grenade launcher. These two were more than enough for me. So if you wanna do the same, I recommend you to buy shotgun's first upgrade and grenade launcher itself with the first upgrade. Shotgun must be bought till level 2 as there is a hidden shotgun in the wall and grenade launcher with upgrade must be bought till level 3. If you do all right and find all the secrets ahead, you will have fully-upgraded shotgun and grenade launcher without spending much money. Through the 7 finished games, I found 4 shotguns, 2 grenade launchers, 4 miniguns, 2 rocket launchers, 2 disk launchers, 1 plazma minigun and 1 magma minigun. In order to get the most out of all these upgrades, you have to buy the weapon and it's upgrade(s) before you find those, so that found upgrades could cover the highest prices. For example: there are 2 grenade launchers can be found in the game, so you have to buy the grenade launcher and it's first upgrade before you find the first grenade launcher.