From The Heroes of Might and Magic III wiki

A place, in your town, where you can trade surplus resources for scarce ones.

If you have many marketplaces you get better trades:

BR - Basic resources (Wood, Ore)
MR - Magical resources (Crystal, Gems, Mercury, Sulfur)

Marketplaces	Gold->BR	Gold->MR	BR->Gold	BR->BR	BR->MR	MR->Gold	MR->BR	MR->MR
1 Marketplace	2500->1	5000->1	1->25	1->10	1->20	1->50	1->5	1->10
2 Marketplaces	1667->1	3333->1	1->37	1->7	1->13	1->75	1->3	1->7
3 Marketplaces	1250->1	2500->1	1->50	1->5	1->10	1->100	1->3	1->5
4 Marketplaces	1000->1	2000->1	1->62	1->4	1->8	1->150	1->2	1->4
5 Marketplaces	833->1	1667->1	1->75	1->3	1->7	1->150	1->2	1->3
6 Marketplaces	714->1	1429->1	1->88	1->3	1->6	1->175	1->1	1->3
7 Marketplaces	625->1	1250->1	1->100	1->3	1->5	1->200	1->1	1->3
8 Marketplaces	556->1	1111->1	1->112	1->2	1->4	1->225	1->1	1->2
9 Marketplaces	500->1	1000->1	1->125	1->2	1->4	1->250	1->1	1->2

You can give resources to enemies or allies.