I'd just get one from a breeder or at a reptile show. In general, Petco doesn't take very good care of their Leopard Geckos (or most of their reptiles, for that matter). I think that it would be better to pay a little for from a breeder, but to not risk having to pay for tons of vet bills if the one from Petco ends up being impacted, has MBD, Crypto, Parasites, etc.

Are they housing it on sand, or a solid substrate?
What size cage was it in? Was it housed with other animals?
Did it look healthy? Was it alert?
Was their a bowl full of water in the cage?
Does this Petco have a history of improper care or sick reptiles?

In the end it's up to you, but I'd definiely be cautious if I was buying an herp from Petco... If you do get it, have an experienced herp vet check it out before you bring it home.