Hi and welcome. I suspect the answer is the 'risks' are the same wherever you buy a used car from. Reputable car sales have a reputation to protect irrespective of the make/model being sold. As with all retail outlets some are better than others so you need to do your own research and talk to friends family etc who may have had dealings with the dealership selling the vehicle you are interested in. I don't think it makes any difference buying a Mazda from a Kia dealer other than they would probably prefer to sell you a Kia? Caveat Emptor.