Originally Posted by
meanwhile
Do you have the option of taking them to a small claims court? Ask them to put in writing that they consider the damage to be wear and tear, acknowledging the period you owned the bike for. Then take them to court. Wear and tear has to occur at a reasonable rate. Unless there are special circumstances - eg these rims are not sold as suitable for cross - then you should have no problem.
It would be a waste of time. No rims are "suitable" for deliberately and repeatedly riding through mud and sand.
Wearing out gear is part of cross, for better or for worse.