Most weapons are untradeable when fully built and those that are tradeable, become untradeable once they earn any affinity (experience). You can't trade weapons, as a basic rule. You start the game with 50 (?) plat I'd suggest spending it on a warframe slot (20) and 2 pairs of weapon slots (24). The only way to get more is to buy them with platinum. So yeah, there are a limited number of inventory slots you get.
