While he could do that, what you can then do is mark him as a non-shipping seller, which seriously damages his reputation and ability to sell on bricklink. If he is an established seller, then it really isn't worth risking that on his end. Also, this is why paying via paypal is the best option, because they can get you your money back without much of a hassle should he not send you your stuff, with a bank it's a bit harder. What I would do if you are really worried about it, is ask him to cancel the order, and then simply re-order the item, so that then your payment falls under the seven days to pay deal.
Regardless however, if the seller agrees that its okay for you to pay him late, then it still falls under bricklink's protection rules, and you would have his bricklink message or email response to prove it, so I wouldn't worry too much.