Ethical business practices ?

Purchased a secondhand "tested" shock for a 50hp Mariner and found it totally non operational due to the fact that the oil seal was defective to the point that half the oil leaked out at some point before it was "tested" and found ok. They refused to give my money back due to the fact that it now has no guarantee as they did not fit the part to the outboard. This was not disclosed at the time of the sale (R6500.00) as I was assured that it was fully operational. BEWARE OF THIs BUSINESS !!!!!