The OP isn’t asking for troubleshooting help...the bike has already been traded. The question is how to protect oneself on a future purchase. If the LBS didn’t help make the previous purchase right, I would go to another bike store in the future. One post suggested an extended test ride, which seems like a good idea.
The only other way to protect yourself would be to purchase with a credit card. If there’s a problem with the bike that the store will not correct, your purchase can be disputed with the credit card company. I’d view that as a last resort though, because if a problem is barely perceptible, and the bike store can’t duplicate the problem, they are not going to be pleased when you dispute the purchase. It’s a burned bridge type situation.