Add-on or accessory lenses vary wildly in optical quality, so one must be very careful in making a selection. Those found on e-bay are very low in optical quality. I am afraid that you would not be very happy with thoses lenses.
I am not pushing Canon's accessory lenses. I am just sharing with you that I did purchase lenses from e-bay and they were terrible. I just don't want to see you unhappy with the product and out the money.