Selling works in any custom quantity, but no quantity of anything when trying to buy will work.

I've tried completely deleting the data/config/plugin and allowing ALL of it to reload and repopulate and nothing's working even when changing from economics to scrap, but it's ONLY for purchases.

I had the same issue, but as a temporary workaround I applied the following change:
(This is just a quick fix for now)

Around line 5076:

[ConsoleCommand("shop.buy")] 
private void ConsoleGUIShopBuy(ConsoleSystem.Arg arg, ShopCategory shopCategory)

↓

[Command("shop.buy")] 
private void ConsoleGUIShopBuy(ConsoleSystem.Arg arg, ShopCategory shopCategory)