i had some PM exchange with the OP, he resides in canada. this means issues with buying a percussion gun, whereas there are no formal regulations for flinters. the gun will be for sport/target, not hunting. i recommended going flint instead of cap, and a .50 would do him well in either a GPR or trade gun, for his intended usage of the gun.
*IF* that USED trade rifle is worthy (aye, there's the gamble unless one has it in hand and knows just what to observe!) that IS a good price, indeed.