Nothing unusual about sending the parts in before you are paid, they would be silly to do it any other way.
When you make a decision to use them, you understand the process, and the time committed. As you indicated, they tell you up front how it works. With them you have a known buyer, at a known price. When you try go sell these on an auction site you are taking a risk, will the buyer pay, will the item get "lost in shipping", and many other variables. Some people are happy with a bird in the hand...
If someone has something to sell, and they don't make a little effort to determine the worth, that is in no way the fault of the person or business buying the item. If you are desperate for money, it isn't their fault either.
Still missing how they have done ANYTHING wrong here.
Good luck to you selling your parts, I hope it goes smoothly for you.
I have to go make an order with GPC.
Your choice, I will not do business with them again. To many other parts places showing up online to be ripped off by them.