Selling parts

Lets say I have an item listed for sell in the classifieds. I get a questions on the item from member A, then member B says they will buy it. At this point who should I sell to?

I'm thinking the first guy to say they will buy or indicates they are interested in buying my parts gets the part. If your interested in buying a part in a classified that should be said first then ask the questions before sending money.

Right or Wrong?

Slow day at work. lol...