Question about pets

When I first started, I was able to buy pets in the second or later towns to corrupt the pets into stronger versions and had no issue doing so in multiplayer.

In recent runs (Not running any madness levels with my less experienced friends), we haven't been able to. It just says sold out over the pet. Is this a setting or something that I have changed? I can't figure it out. I know Madness levels will impact whether later pets are available, but that shouldn't affect Maness 0 runs.