Based on the rules, it looks like the 3 buys and 3 sells are in addition to the ones you get in the Transaction phase. It's not totally clear, but at the end of the First Contact phase rules it says
The culture is now open for trade with any player who lands it its system, including the player who just discovered it.
I'm not sure this is going to be a huge difference over replacing it. Having 3 buys and 3 sells is already a lot considering this culture hasn't been discovered (I'm thinking this would be relatively early in the game). Also most cultures will be discovered by landing at a city, so you'd only end up with an additional 1 buy and 1 sell anyway, for a total of 4 each.