Well, sadly trade of equipment wasn´t included in HOI2, which is too bad since it is an important and accurate feature of the world market and diplomacy. You can see minor countries using weapons like Sherman or Allied aircraft, so where do they come from? Licensed production?
If it was implemented (which I really hope for), you should be able to trade equipment for money (obviously), own equipment (exchanging equipment would be interesting), resources, supplies, provinces, and blue prints. It could also be deals like: "I give you 2 battleships if you agree to join my alliance" or "Give me all your tanks or I crush you like a bug!". This means that minor but resource-rich countries like Persia would be able to sell some oil to buy some advanced equipment, which should be interesting. Of course, the relationship between the countries trading is important; you don´t want to arm your enemies.
If you sell a unit, all the manpower from the unit is regained, so the buying nation must of course still have manpower enough to build that division.