I walked away from buying a 1989 Nissan Maxima with 133,000 mile, for 1,500. Did I do the smart thing?

No way. I have had mine since my junior year of high school (I am in my last year of college) and it runs better than any new car out here. The thing is the engine runs like it is built by the gods.

my maxima is a pos and still runs good

Hard to tell w/ just that info. I never put a lot of stock in mileage by itself. $1500 is ok for an '89 car. It depends on how the car was driven, where it was driven (commuter, many dirt roads, roads w/ potholes and other rough places), and what upkeep has been done so far (timing belt, frequency of oil changes, how dirty/oil-covered is the underside). I've got a friend who's on Craigslist and has found 2 trucks (94 Chevy 1500 and 92 Toyota Tacoma) around $2k, but both have body damage and one has a bad head gasket. You may have found a great deal, but you may, or may not, have had problems come up in a few months.