shifty

i was thinking about purchasing a used slk(2001). i was wondering if the dealer will give me crap because im not the original owner
also does it seems very strange that the 2001 i found on one of those search engine was only 31,900 for '01 with 15k miles
do slk depreciate that bad?
also whats the difference between the certified slk's and ones at those used auto lots?
thanks