Thanks for the message Polarius, and welcome to the Pit! There are two reasons why you might not be able to sell a gladiator:

1) As Aphexil wrote earlier, if your gladiator is currently fighting in the Blood Games tournament he/she can't be sold.

2) Your stable doesn't have enough gold to buy a new gladiator if you sell your only gladiator.


Good luck in the Pit!

-Nate-