At the point that someone is stealing chickens and taking up residence in abandoned mines while wearing spooky robes, one might be forgiven for not assuming their choices are rational.

An argument for not buying a chicken could be made due to that creating a witness, but I'd argue the odds of that actually mattering are much lower than the odds of getting caught trying to steal a chicken.

The only scenario in which I see buying the chicken MAYBE coming back to bite them would be if the heroes came in, grabbed the decoy chicken, left without being stopped, and actually presented the decoy chicken to the mayor.

Even then, the party would probably just go back to the mine, and likely bring some of the town guard with them.

In short, yeah. Just buy the decoy chicken.

Anyway, Ellen is trying to mimic holding a chicken with one hand, and petting it with the other. If it does not appear Ellen is doing that, that is her fault. It has nothing to do with me, who expertly drew her failure.