Being the seller, how do I allow presale veiwing?

Hello vin.p

Easy answer here. You can't.

All you can do is encourage your buyer to ask lots of questions, you add lots of photos, and an accurate description. 

You might like to also say that there is no obligation on the buyer if they purchase it and decide they don't want to proceed after they have inspected it at pick-up time.  

Thank you.