These have all been great questions. I would add a few more questions/points:

1. Is the snake a wild caught (WC), captive hatched (CH) or captive bred (CB)? The answer might be obvious from answers to your other questions, but I would ask anyway.
2. Get lots of contact info from the vendor you buy from; phone numbers (cell, home, business if they'll give it), email address, web site, etc. You never know what might come up after the show and when you need to contact them you realize you can't.
3. Get an invoice identifying what you bought and marked "paid" from the vendor you buy from. This will help with your record keeping and if you need to invoke any gurantee. I offer an invoice when I have a table at a show and you'd be amazed how many buyers do not want one.
4. Have the vendor sex the snake in front of you before you buy. Even if you do not plan to breed, it is useful information.
5. Walk the whole show and look at all the vendor tables before you make your decision.
6. Look at the vendor's setup at the show. Is it clean? Do they have business cards prominently displayed? Are the people behind the table helpful? Do the reptiles available look well cared for?
7. If you buy with a credit card, watch where and how your card info is used/written down. Many vendors do not have the capablity to swipe a card at a show so they have to call it in and then write down the key info for later "batching" to the credit card company. I would want to make sure that my info was secure, out of sight behind the table.
8. Look up the web site for the show; many times they will have the vendors that will be at the show listed. Do a few BOI searches on ones that look interesting. Many times the show's website will have links to each vendors website - another good source of info. You also need to understand that the shows do not usually ban a vendor even if he has a very poor rep.

I hope these help. As a vednor, I would never be affended if someone asked questions - in fact I'd be thrilled that they had done their homework and I'd be less apprehensive about the future of my babies!

Good luck and make sure you post pics of what you buy!