I have owned my own business for nearly 30 years and make monthly sales tax payments so I have some experience with sales tax.
Prior to bidding I carefully read all of the information on GP's website about the fees and taxes.
I purchased a truck from GP 18 months ago. It was my first and only purchase from GP. The invoice I received from GP showed an overcharge for sales tax. I attempted to get GP to correct their invoice to reflect the correct sales tax. I made many phone calls to GP and to a couple to the Department of Revenue in the state where the sale was taking place. The man from the Department of Revenue was even willing to talk to GP's tax department but GP declined. Even though I could show GP in writing the sales tax I was being charged was wrong and excessive they would not correct it. They did offer to void the sale and put the item back up for auction or I could pay the invoice with the excess sales tax, and after the vehicle was registered, and I showed them that I paid less sales tax to register the truck, would they refund the excess amount. I chose to complete the sale, pay the full invoice from GP and then register the truck (paying the correct sales tax in full) then send in the proof of correct tax payment to GP. Then wait.... about a month or maybe a little more. Only then did I get a check for a full refund from GP for the sales tax I paid to them. It is possible to not pay the sales tax in full a second time to your state Department of Motor Vehicles if you request the "proof of sales tax paid form" from GP. I forget what the form is called officially. You must request this from GP because they do not send it automatically.
What I learned from this (and what GP never was willing to admit) is that GP will charge you every possible tax that could possibly be applied to the sale so that they could never be found to have failed to collect sufficient sales tax. If they failed to collect the sufficient sales tax then they would owe not only the balance due, but also fees and interest. So they seem to be excessively cautious and charge any and every conceivable tax weather it applies or not. If you buy a vehicle and pay the GP invoice and then register the vehicle you can jump through the hoops and get back any excess payment. However, if you do not register the vehicle or you purchase merchandise from GP then I am not sure how you can get back any excess payment of taxes. I wonder what happens to all that excess tax revenue that is collected by GP but not refunded?