Not replying to anyone in particular...

It can also depend a bit on the type of items you sell, and the questions that are commonly asked about them, in so far as whether or not being able to answer ASAP will cost you a sale.

Personally, I rarely get "sold pending the answer to this question" types of messages. Sometimes people do want additional or clarifying information before they buy, and most probably think they're asking a simple "yes" or "no" question, but from my perspective they rarely are, and I like to take the time to send a thoughful and sometimes comprehensive reply. 

A classic example is "do you combine postage?", I won't just say "yep", I'll explain how to get it to work in case they're using the app or summat (that one is asked often enough for me to have a saved message template, though, so doesn't take much time to respond to that). 

Other questions tend to seem simple, but I've returned essays to them   (eg some people see one of my items and ask something like "can you do this in other colours and/or sizes?", the answer is yes, but to save time on any further messages, it's easier for me to outline several of the main (but actually infinite) options I can provide. 

I have a pretty comprehensive knowledge of certain materials, too, so I always take a bit of extra time to explain some common misconceptions when I receive a question that is obviously based on those misconceptions. It does take me longer, and it means I have to set aside some time to do it (rarely straight away), but I've also earned a few dedicated customers that way, too.