I have tried this on a few vendors including buttercup and silver in ponydale; the NPC shop checks to make sure you can afford to buy 10(available bits in inventory) of the item but then only charges you for 1 of them while still giving you the 10 in your inventory. 10 for the price of 1? Good deal!
The sell 10 feature works correctly which makes it now possible to sell back the 10 items for a large profit. This can be used to make infinite bits.