The Market gives your village a certain number of merchants depending on its level.
Merchants can advertise trade offers, accept trade offers, and send resources to other villages.
Most players do not build the Market above level 20, as the increase in Merchants (125) is not worth the added farm spaces used (471).
In the beginning of your village the Market is useful for trading excess resources for those you are in need of.
After you have several villages, a level between 15-20 is quite useful for sending resources to developing villages.
Table of Merchant Build Costs, Population Usage, and Merchants taken from Tribalwars Help