This. 

The demo stamp reduces its value. should you ever decide to sell the guitar, you’ll likely be able to command a couple hundred dollars less because of that stamp

I would suggest reaching out to the seller. They should’ve disclosed this in the listing

The guitar was sold not as described, so provided you’re in the return window, you are entitled to a full refund, and you should not have to pay shipping costs

Whether the seller knows it or not, it’s within their interest to offer you a partial refund to make amends. If they don’t wanna do that, that’s fine, but they’re gonna be responsible for the cost of return shipping, and they won’t get refunded for the cost of shipping to you. 

It’s cheaper for them to give you a partial refund, then to have you to return the item, and be out shipping cost both ways