First you need to check them out before asking any questions of them.
Putting their name and the word "review" or "complaint" into a search engine is a good start.
Then look at the way the items are represented. If they sell diamonds, which lab reports do they suggest?
Are their prices too low? Too high?
If they pass those tests, stat asking them directly
Ask them about what happens if you're not satisfied?
Is there money back or satisfaction guarantee?
Idj. I've purchased from them previously, but what I'm looking at now is a much bigger purchase. Once I buy the item, I want to avoid shipping it back and forth for any changes. I don't want to regret not asking a specific qs beforehand. Also, it will be a custom piece so I can't return.