Alright, so, 2 things.
First off, the normal percentage works on the "buy 1" modifier.
But the "Buy 50" modifier just times the current price by 50.
Meaning you're spending a LOT less for a lot more.
And the "sell" modifiers..
Selling 1 and selling 50 work just fine.
That's the thing.
Get enough to buy 50 (with the modifier) and then sell 50.
Then repeat, you should get tons!
And hey, game devs, if you want to fix this, just change your sell price to 0%.
This should definitely be fixed.