Invite them to sit or have something to drink, and tell them you'll be nearby to answer any questions they have. If they say they love your work and you are uncomfortable talking about it, you can ask them a question in return. Instead, think of it as a fresh perspective on your work. If you think they're missing a crucial detail about content or process that would explain the piece or make it more interesting, share it with them. Be charming and interested, not pushy.
People are often excited about art but very insecure about their lack of knowledge.
Selling If someone seems very interested in buying a particular piece, try to satisfy their concerns. Do not let them leave without a photo or slide of the piece in question. In any case, send a note or call them within a week. Thank them for coming and ask if they've been able to make a decision yet. You should never be alone while your studio is open to the public.