@justicejej Sorry to hear that, dude. Afaik the buyback option has a limit on how far back you can go to recover an accidentally sold item. Off the top of my head, I think it's the last 20 things you've sold? Someone from SWTOR support/one of the veteran AHQ peeps will have a better idea on that though.
If you can't see the undertunic in the buyback tab, and you were selling a lot of stuff to the vendor at the time, this might be what happened. The other possibility that comes to mind is that in the middle of selling off gear, you may have dragged the tunic out of your inventory accidentally, and thus destroyed it.
The best thing you can do is contact the in-game team, because they'll be able to a) help you figure out just what happened and b) see if there is any way to recover the gear. Here's how to reach out to them for help - or if you're a f2p player, you may be able to contact them via email (check here for more info on that). Best of luck - I hope you manage to get your RP gear back!
- RB