Buying over the internet from 1000's of miles away need to be sure the seller/consigner is the real seller/owner is not some scamer just reposting some other car advertisement from the web.
I note you have had some one to physically look at the car. Take consideration of the comments
Very likley such a car would be know to Buick Club of America members, even in northern Michigan.
Convertibles are desirable so prices get "reduced" for a reason .
Extract below is from the BCA 2005 Judging manual. For 1948 does not list "L" (Linden NJ), but that may be a BCA error.
The first digits engine serial do not represent the model year. Some times they do, but more by chance.
The series is stamped ahead or after the main number: it changed to after for 1946 production.
The engine in that car is not 1948. But not unusual or a concern to many people
The engine number stamping does not look right being on top/right of pad. The stampings I have seen are more in the centre of the pad. Stamping of the series not always in line, but have seen one that far out of alignment.
I would be wary where the ID tags are secured with cheap rivets from a hardware store. Data tag's never had washer/river to fix it in place
Keep looking:- will be other 56C in the market at some time