The restaurant is a chain franchise. In the script it's called "Dixie Doug's" and it's meant to give the impression that it specializes in pie from the Southern United States. Giles is eating Key Lime Pie (which originates in Florida).
The guy running the restaurant (Pie Guy in the script) is putting on a fake Southern United States accent, hence the use of "y'all" and other cliche'd phrases, despite the fact that really he's from Ottawa Canada. He's doing this to make it appear authentic to his customers.
The phrase "bought it" isn't meant to literally mean to purchase something, it's common English slang to mean:
To believe in, especially wholeheartedly or uncritically:
Giles is saying that he believed the accent and the restaurant to be authentic. Whether he honestly believed it or not is uncertain, as he was attracted to 'pie guy'.