on facebook there is a generic reply of yes it's available or no it not available just above the reply line. click on either bubble and it reply's and auto sends..

the is this available is a generic question that pops up when you hit the message bubble in your listing. basically a 1 click effort for the person sending the question. it's perfectly normal.
the part of facebook marketplace that bothers me is the brain surgeons who reply interested in the listing. you now have to click on his name and message them directly so it's 3 times the effort to respond to them that way than if they messaged you interested in the item or just hit the is this item available.