I'm going to buy new items, switch them in the box with previously owned broken ones to return for my money back
What are you trying to return?
I broke something for my girlfriend and can't afford to replace it.
It's really rude to return an item you broke. It's also considered theft, so you can get in trouble. You are also raising prices on everyone by doing that.
I don't think it's rude if I can't afford a replacement any other way.
It is still considered stealing to return broken items and replace them with new items. You should just replace it after you save up some money.