It has nothing to do with spiritbond ;;
It's bound to the character after one use, no spiritbond progress at all; a clearer example might be furniture: there is absolutely no spiritbond involved, and as you place one furniture in your house, that piece becomes bound to you.
See the difference in the spelling:

spiritbond -> spiritbonding, as in make a bond between you and the piece of gear until 100%, then it becomes convertible
bound -> binding, as in bound to you because you've used it, therefore not suitable to sell or trade since it's "second hand"

Now read the OP again