As RITZ stated, replying to ads in the classifieds is not allowed since it would result in an early bump (which is against the rules
If you see a classified ad in the paper you're interested in do you take out an ad the next day to inform everyone you'll be phoning the guy? There's zero need to post in someone's sales thread: contact the seller via PM. If you want a confirmation your message was read use the confirmation option (the "Request a read receipt for this message" checkbox at the bottom of the page when you're writing a new PM).
Also remember the seller may have been in talks with someone else before you contacted him.
Lastly, why was this posted in the Airsoft Media section? Moving it to the newbie tank. (also displeased tsk tsk)