People normally wear my merchandise.
I join things, I separate things, I am in the things.
Who am I?
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Sign up to join this communitya zip (or possibly velcro)
because
a zip joins things together when zipped up, and separates things when unzipped.
a zip is worn as part of clothing.
Perhaps you are...
Thread?
"People normally wear my merchandise."
People purchase and wear clothing, which are made of threads.
"I join things,"
Thread can be used to tie things together, thereby joining them.
"I separate things,"
Thread can be used with fabric to sew a pocket. Things inside a pocket are separated from things outside the pocket.
"I am in the things."
Threads are in clothing.
You are
Cleavage Couture, makers of the Cleavage Couture Bra.
People normally wear my merchandise.
Well, yes.
I join things, I separate things, I am in the things.
cleavage n 1: the state of being split or cleft; "there was a cleavage between the liberal and conservative members" [...] 4: the line formed by a groove between two parts (especially the separation between a woman's breasts) 5: the act of cleaving or splitting cleave v 1: separate or cut with a tool, such as a sharp instrument 2: make by cutting into 3: come or be in close contact with; stick or hold together and resist separation
so that "cleavage"[5] via "cleave"[3] means cleavage joins things;
"cleavage"[1] separates things, and
"cleavage"[4] is what you're involved in.
(I suspect your riddle is too underspecified and may be Too Broad.)