Highlight the number of stock you have for the AK-47. Delete that number and leave it blank. When done, move your pointer over to the number of guns you want to buy and sell and choose any number. Then click "Sell" numerous times and watch your cash level rise. Your cash level rises because when you deleted the number of stock. The game does not know how many guns of that type you have. Note: This only works on AK-47s.
Various Steam Achievements
Achievement
How to unlock
The Pathfinder
Successfully beat the first set of travel challenges.
Top 10 Most Wanted Arms Dealers
Make it onto the list of the top 10 most wanted arms dealers.