can a policeman arrest a ex felon for possesion of a fiream if he sold same to him?

There are two scenarios where such an arrest would be legal.

1) The police officer did not know until after the transaction that the person was not qualified to own a firearm due to a felony conviction.

2) The police officer knew of the person's felon status. However, the person initiates and pushes for the transaction while the officer only provides the merchandise and possibly the venue for the transaction. Such a scenario would not be considered entrapment.

Yes. If a cop is selling a legal firearm and realizes an ex felon is the one who purchased it then yes he can arrest them or report them to the local police.

As long as he didn't know that he was a felon at the time of sale. If he did, then it would probably be entrapment.

Sure, in a sting.

But the felon prob claim entrapment.