It absolutely is stealing if you return a $10 item for cash or credit that you never paid them for. Because their wholesale cost/value might be around $5, but they gave you back the $10 retail cost. So you stole $5 from them, plus they're out all the labor to deal with return/restocking/reselling. There's no grey area, it's theft.
If you get store credit only then it depends on the margins of all the items returned and exchanged but they're still out the labor costs no matter what.