"Buyer beware" ?
I'm pretty sure you're wrong on that front. Has stated in the following link:
https://www.gov.uk/marketing-advertising-law/regulations-that-affect-advertising
The Consumer Protection from Unfair Trading Regulations mean you cannot mislead or harass consumers by, for example:
Including false or deceptive messages
Leaving out important information
Using aggressive sales techniques
Has @thegummy says, "Are you saying they could call it an American Fender, it really is a Harley Benton with the logo changed and it's still up to the buyer to notice at the time, otherwise that's perfectly legal?"
If it was bought through eBay and paid with PayPal, they could very easily find in favour of the buyer. If he can prove that in the original listing there was no mention of a pickup change and the bass was advertised as stock, then this is false advertising and a refund would be issued; presumably the bass would need to be returned to sender.