Let's say you have Copper/Festival/Haggler/Green/Pot. If you play the haggler you can get festival/X - wahoo. If you buy the vineyard as well - enjoy your free copper.
It is also a drawback if you run out of actions below a certain price point. E.g. if you stock up on pearl divers, then haggler & buy a menage means you have to take a copper once the pearl divers are gone. It is also a drawback if you are playing golem, Tr/Draw, or if you need the haggler cash (e.g. you draw just haggler & want a pearl diver or you draw Haggler/Pot & want a scrying pool).
That being said, Haggler is a very effective gainer. I'm not sure if there is any true synergy here though. It seems to be a linear addition. Still it can be a quite strong setup, particularly with enough cheap useful actions (e.g. Xroads is great here).