Simply because a police officer was a seeming seller doesn't mean that his role played a determining factor as to whether the defendant chose to be involved in the transaction. If so, then any time an undercover officer makes such a "sale" it would be entrapment -- and we know it isn't the case.
The easiest way to explain this is, e.g. where someone who has never been involved in using marijuana lives outside of town in a quiet area. An undercover police officer hangs out in front of the defendant's property and waits for him to leave. The person has a friend over and both leave the house. The officer then calls to both and inquires whether they want to buy some leaf. The friend says he'd like to purchase. After discussing business, the friend says he doesn't have enough money and leaves. The officer continues by asking the defendant. Defendant declines. Officer persists and says that nobody will know, it's a remote area, etc. That's closer to entrapment.
In your scenario much depends upon the circumstances, e.g. whether the officer came uninvited onto private property and just solicited merchandise. As stated, however, this is an affirmative defense which means that the defendant must prove how he was "entrapped."