Recommended strategies for Mercante in Fiera?
There are only two strategic decisions over which a player has control: the amounts bid in auction and the quantity of cards traded. A winning strategy requires paying less on average for one's cards than (all) the other players; and then having a bit of luck. 

So, based on whatever patterns seem to be discernible in the merchant's average choice of how many cards to auction off, only bid less per card than the expected pot divided by the total number of cards (40 or 55 depending on edition) in play. Posture all you want to mask this strategy, but adhere to it strictly.

Second, never trade down on the quantity of cards held. Again, mask this strategy by posturing with and to the other players, but adhere to it strictly. You want to posture as an "active trader", in hopes of occasionally leveraging another player's superstitions to trade-up on card quantity; but don't expect  doing this more than once or twice an evening.