Unless you have a listing for single units, then they would have to contact you to have you set up a special listing for them.
Or you can list sets of 4, 6 and 8 (or whatever quantity you wish) in individual listings.
You can see how I set up the ability to purchase different quantities in my soap dish section
http://www.etsy.com/shop/crochetgal?section_id=5694357I offer from 1 to 90 units in varying quantities.