You can't "trade" your Regkey, but if you follow these steps you can get a free head texture slot, and will get more items in the future:
1) Click on "User CP" at the top of the page.
2) Click on "Edit Profile".
3) Scroll down, and put your Regkey into the "Toribash Registration Key" box.
4) You should now receive a free head texture slot with which you can upload a texture to the game. Click on "Torishop" at the top of the page, then "Inventory", then activate the item. You can now click on "Textures" and upload a head texture. If you want more info about head textures, click
here.
If the item doesn't appear within a few hours, mail support on
[email protected].