Before the Ferengi encountered the Federation, they were already an empire whose primary interaction with other alien civilizations was profitable trade. This culture of trade is embodied within their [Rules of Acquisition][1], which are a series of cultural mores built around what the Ferengi consider policies of successful, if not less than honest, trade.

Their interactions with the Federation would be more challenging because many of the more advanced races of the Federation would have replicators, devices capable of creating material objects using replicator templates and source matter. Replicators can create anything that is stored as information within their databases. As a result, a person can get anything that has been described and stored as information. It would seem that people in the Federation would have no use for trade, but that is a misnomer.

Replication still requires energy which must come from somewhere, even in the energy-rich Federation, replication credits were part of most people's lifestyle. These credits were not money, per se, but it implies that replication of materials still has some type of cost associated with it.

But one of the reasons a person might not want to use a replicator could be based on the idea they might want a particular product not in the Federation replication database. With thousands of worlds, and trillions of potential artisans, there is always likely to be someone out there producing a work of ART that is not within the replication database, made by hand, with all of the associated psychological baggage associated with making something personally. Technically, any work of art could be deconstructed and recreated perfectly but it would lack the touch of the artist beyond what was originally created. (Yes, you could also use a computer to alter each piece, so they appeared to be unique but this would still not be a work of art.)

However, trade in materials that are difficult to replicate, heavy metals, synthetic compounds, complex molecular structures, delicate organic compounds might still be traded by the Ferengi to races within the Federation, from species outside the Federation who may possess technologies or capabilities beyond current Federation science, art or technology. 

Trade is not dead in the Federation, it simply is more complex due to the inclusion of a science (replication) that allows simple objects to exist with no more effort than modern mass production. This still leaves plenty of room for artist expression, scientific advancement, or cultural drift to create new expressions for the Ferengi to trade in.

The Ferengi, of course, would prefer to trade with races who do not possess the ability to relatively easily, reconstruct matter into any shape they prefer, since it reduces their ability to create commodity markets where profits can be standardized and controlled/exploited. 

The Ferengi consider the Federation as a whole, particularly where replication is easily available, a poor customer. Treaties with the Federation ensure there will always be somewhere replication is not easily had, thus allowing the Ferengi to create some Profit.


While it is not one of the known Rules of Acquistion, the Ferengi might say "Trade cannot be replicated."


  [1]: http://www.sjtrek.com/trek/rules/