Ask Mark if he'd be willing to give a good discount on the parts one'd need to upgrade it to the AICS PCS state to either you or whoever buys that rifle? Probably best you can hopeful right now.
I think you should ideally get a full refund in the form of store credit. Considering the current state of airsoft availability in Canada, I doubt Mark will have trouble selling that rifle. However, I doubt that (full store credits) is going to happen. The 3 months really does hurt your case.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."