Bind System?

So this might be a dumb question but it seemed weird.
So i bought a +10 Sentinel Passionate Leggings from someone and he COD it to me. The thing is, it was not bound to him while it still had 3 unbinds left. Of course, when I bought it, it became bound to me.

My question is, how is the item not bound to the seller? I still have the item bound to me with all 3 unbinds left. The bind system is a pain for selling and buying, and I thought if it was someway possible to bypass the bind system.
From my knowledge, as soon as you enhance something over +8 (with no enchants) it bounds to you. (well it at least happened to me when i enhanced Blood letters to +9)

Also, I wanted to ask how do you know when something bounds to you? Like i said before, I know +9 or more on no enchants bounds to you, but what about the enchants? I'm sure it has to do something with the level of the enchant, but is there some kind of formula for it? I hate it when i start enhancing and enchanting things, and it becomes bound to me while I have no real use for it.

PS. I want to sell my +10 Rounded Sentinel Passionate Leggings (Yes its rounded now).

Thanks!

The seller probably got the item from a Surprise Supplies. Those come unbound.

The level of enchant that items start to bind is at r9. You are correct about the enhance levels (over +8). Other items that bind when traded are 5* items.