Im new could use some help

How do i stock merchan bulidings,

and while im at it i was wondering if i could somehow set lets say a single black smith building to allow players to upgrade thier individual armour, lets say player one walks up to the black smith and it becomes his (capturable) so he can upgrade armour, than player 2 comes and it becomes his (capturable) and than he can upgrades his armour , does that make sense ?

Stocking Merchant buildings is done through the Unit editor. Find the Merchant building and the 'Items sold' or 'Units sold' fields, and then put whatever items/units you like in them. You can edit costs and stuff in the unit editor too, but you do that by choosing the unit you want to change the cost of.

For the second one, use an action to make it rescuable ('Unit - Make unit rescuable' if I remember correctly). Do this on the blacksmith. Then put a location around it and make it so that when a unit leaves that region ('Event - Unit leaves region'), and then a condition to check that the leaving player is also the player that owns the blacksmith, and then the actions are that the blacksmith is returned to a neutral player and made rescuable again.