Here are a couple...
1st: Most important, run a Board of Inquiry (
link here) search on the seller before you waste your time asking them a bunch of questions.
Once they have successfully passed that stage then...
2. What is it eating, "live" or "frozen/thawed". Depending on your ability to get rodents this will matter. Frozen/thawed are typically easier to get, so if a snake is only a "live" eater, you need to consider that fact.
3. Temperament?
4. Their shipping policies, warranty?
5. How do they take payment?
6. Captive bred, captive hatched or Wild Caught snake?
7. Talk to them by phone at least once, this serves many purposes.
odds:
8. How long have they had the snake for?
9. Why are they selling it?
This is a start, good luck.