Hello, Mechanical Man. Here's my trade suggestion for your map:
All units have a 1-slot inventory (can use items). You have, say, a horse armor with a price of 150 gold.
Should one cavalry units purchase one, armors will be given to up to 5 more cavalry units within 400 range. If there are less than 5, the price for the missing ones is refunded. So if the bying unit and 3 more hussars are in range, there will be 2 armors left and 2*25 = 50 gold will be refunded.