im fairly certain that any product bought has a 30 day warrenty on it, even if its second hand and you've signed a seen as sold form.
If you haven't signed a seen as sold form then you have bought the marker in good faith that its in fully working order.
If you have a copy of the email he sent you then you have every right to demand your money back or you can have it repaired and bill him for it.
So warn the guy that you will have it repaired if he does not return your money and the invoice will be going to him
(probably cost more than the guns worth)
this is all on the assumption that in law this is treated the same as second hand cars.
You have every right to deman you money back is the bottom line.