Are the rules of Dominion permissive, or restrictive? Do they say "You may play action cards during your Action Phase", or do they say "You may not play action cards during your Buy Phase, or your Clean-up Phase, or during other players' turns"?
In this specific case, both:
"Generally, a player can
only play Action cards during the Action phase of his turn. However, Reaction cards are an exception to this rule as they can be used at other times."
That said, the rules are generally interpreted as restrictive in the sense that only things written in the rules are allowed. This has come up multiple times when it comes to point counting. Sure, the rules don't say you can't sit there with a sheet of paper and count points as you play. The rules also don't say you can't play a card face-down and claim it's a Village, or that you can't look at the other players' hands. But I know of no other card games that allow any of those unless it's specifically stated in the rules (Bullshit, for playing face-down; 3-5-7 poker, for seeing other players' hands), or overruled by a specific card (e.g., Possession for seeing another player's hand, and I'm sure there's at least one in Magic).
Similarly, most games don't allow table talk or politics unless specifically allowed in the rules--even Intrigue (not the Dominion expansion), which is all about bribing your opponents and convincing them not to screw you over, has specific rules for table talk.
In other words, plenty of rules are implied by convention, including those against bargaining for points. More stringently, the game designer has stated that Dominion is designed to be as free of politics as possible, which also implies that bargaining during a Masquerade play is not allowed.
Now, again, what you do
at home may be different, so long as the other players agree. But in tournament play, bargaining over Possessed Masquerades, giving "advice" on Contraband or Possession, giving or asking for strategy advice, are not allowed by the rules as written, and therefore are not allowed during play.