Offering any service (such as pet-sitting or house building) where payment is requested, discussed or negotiated beforehand is a form of cross-trade. This is because there is no guarantee the job will be complete.

You are, however, allowed to provide the player with a gift or reward (and accept a gift/reward if you're the person doing the service) after the job is completed, provided it wasn't discussed beforehand and is given as a thank you rather than a payment.