I noticed this last night and bug reported it in-game, in the hopes of seeing if other people have encoutered this error or have also been able to repeat it I am posting here as well.

I noticed on Monday (I believe) when playing my Dual Blades character that when I went to the tailor I had considerably more options available for my weapon choices then I had originally. It perplexed me for a while until I realized why that might be: I had a number of replacers in the bank that I'd had mailed to me from my alts.

It was a nice touch I thought, until I went to the bank and noticed that all of those costume replacers (including the ones some 12 levels above my character) were now Bound.

Apparently having a costume item in the bank makes it available at the tailor without having ever equipped it or even being able to equip it. However, after having tried the item on at the tailor (possibly just visiting the tailor did it, as all of them were bound to me including some axe ones that I do not remember bothering having tested the apperance of) it became bound to my character.

While in this case I found it more quirky than annoying, I had inteded to try them all anyways, I can see this being quite a nuisance for someone who had items they intended to sell on the Auction House located in their bank only to discover them suddenly and inexplicably bound just because they tweaked the colors on their weapon at the tailor.

I have a gun costume replacer 2 levels above me that I mailed from an alt sitting in my bank.
Just checked at the tailor and it is not available nor is it bound when I go back to the bank.
So it's not a common bug at least.

If I had to guess, you didnt see this item at the tailor because the item was higher level than you. The costume piece can only be unlocked if you bind/equip the item. Since the item is higher level than you, you are not able to equip it and bind it to you.

If what the OP was pointing out really is some quirky bug, then in a couple levels you may notice that custume peice magically appearing at the tailor, effectively forcing the item to bind to you.

Is it a gun costume replacer for a gun skill you can currently use?

By that I mean, is it a Pistol replacer for Pistol skills (which you likely have) or a replacer for Shotgun or Assault Rifle or something which won't show up at the tailor until you have taken that skill?

This may also be a Dual Blades (and possibly Archery and Single Blade) specific error as they don't have multiple categories of items (Pistols, Shotguns, etc) on the tailor page. I'll try to check when I get home from work and have time to finally get my Archery and Single Blade guys out of the tutorial and see if it happens with them.