IMO, preapprovals make no sense. Like @SandyToes, I did it a few times initially, but quickly realised that. Make the guest place a booking request, make sure he/she has answered all your questions and you’re comfortable accepting his/her booking, and then accept. That’s the way to go. Anything else is just out of order, and amounts to unnecessarily giving up control of the situation. And as a host, you don’t want to do that.
Oh, and it’s almost redundant to say so, but in my experience, inquiries most of the time don’t lead to anything, regardless.