Try clicking on the image of the equipment, instead of the "buy"-button.
It will be exploited.
I know because I used to exploit it back when there was no limit ;)
If I have 1 million income and have armored up my guys as much as I can, let's say 999,500 in upkeep. I could buy 1000 FN MINIMI SPW before going into a deathmatch or a big war, and then sell them right after. Being in debt before the income clock actually hits zero doesn't really matter, and it still doesn't matter if you have cash left after buying the weapons if you're not afraid of losing a couple of million.
That's why it's limited, and it will probably stay that way. However... I don't see a reason to why we can't sell more than 100 of something. Maybe a "Sell All" button?
//Formidable - DeathWish MGS