I think it's perfectly legit to ask the seller any Qs you might have as a buyer, stupid or otherwise IF you are genuinely interested and without having to read the entire manual from cover to cover or do 20 hours of research on Google.
Of course, no one has time for time-wasters who are not actually interested in buying.
You'd hope the seller should already be able to answer Qs about the thing he or she is trying to sell and, as one of the earlier posters said, they can always update their ads to reduce repeat Qs.
At the end of the day treat your potential customers with respect, after all it's you who wants their money. Simples.