Caveat emptor.
Legally, the question will be eactly how did you describe the router at the point when you offered to sell, and at the point when the other party offered to buy.
If you did not make undue or untrue claims for the routers capability (in respect to the areas where the buyer is complaining ) or if you made no claims at all as to it's performance then the risk lies with the buyer.
Did you originally claim that the wireless function would operate succesfully over the distances stated by the buyer as being required, or describe in any manner the level of performance that he may reasonably expect from that particular example of that router model.
If you did make statements to potential or actual buyers in respect of performance, did you stress that the performance figures you may have quoted were that experienced on your PC and on your configuration and that the same may not be experienced on his configuration ( here, I think of the words of wisdom posted by waltergmw above) , on his PC, on his ISP and on his telephone line.