An hour and a half isn't too long, especially with those temps. A sack is always a good option for snakes as they feel secure in the dark, enclosed environment. If you can't find a sack, you can also use an appropriately sized sterilite or tupperware type container with a hide. The seller might provide something for you to carry your snake in as well.

First off, you should ask if it was Captive Hatched or Captive Bred, and if so, was the seller the one who produced it? Captive Hatched isn't necessarily bad, but they usually have to be treated when they enter the country and many sellers will not treat them before selling them. Those are usually pretty cheap (under $20) and can have a number of problems such as ticks, mites, or internal parasites.

Spotting mites is relatively easy. Look around the head for small specks, and also in the container they're stored in. If you rub your hand along the snake, you should be able to see some small specks on your hand.

Make sure the snake isn't too skinny. If it's showing it's spine or looks triangular, it is probably too thin. Balling up is a good sign as well as actively flicking it's tongue. It shouldn't be loose or "limp" in your hands. Another negative sign to watch out for is if it is opening its mouth a lot or keeping its head up for long periods. That usually a sign of a respiratory infection.

Ask the seller a lot of questions about the snake. If it looks like a good deal but the seller blows you off, keep looking... it probably won't be a good deal in the long run.

Good luck, and be sure and post pics when you get him/her!