I'm almost tempted to write back.
"Dear M.A.H. Nice watch, I'd like to buy it, but I think we can come to a far better arrangement. Your offer to pay me back $4999.99 as and when you can is generous, but rather onerous for you. And with no disrespect to you - I do not know you - I am running the risk that you will not ever be in a position to pay me back. Rather than have you encumbered by this large debt for the foreseeable future, wouldn't it be better to reduce the sum to be paid back?
I feel that $500 would be both an acceptable amount for me to risk, and for you to carry as a debt here. Can we agree upon that? If so please let me know and I'll pay the $500 for the watch and you can post it immediately. "