There was recently a big update in SM. This is the forum link that explains everything.viewtopic.php?f=403&t=26904
In short, you cannot sell weapons anymore, but you can fuse them in the fusion core to increase the power. After the power has become high enough your gun with "level up" and have +1 damage point (from what I've seen). You can also fuse torsos and legs.
Theres a lot more than just that so read up (: