There should be an option to reserve one of the slaves on sale each day to keep it from refreshing at the end of the day or when you retire a gladiator. This would obviously eat up one of the slots that you'd normally get to see a new slave in, but would allow you to save one if, for instance, you had a glad in the blood games and couldn't retire it for a few more days, or if you wanted to retire a glad and didn't want it to refresh that one away.