I do not like buying or selling in/at auctions either...but the market for transactions becomes really, really limited. I work with two partners in the trade/brokerage of vinatge and classic vehicles and we do work though word of mouth, mainly because there is no (profit) margin if you go through auctions. But it is extremely difficult and certain vehicles are just no there. And the 300SL is definitely one of them. During the last 24 moths, we sold a Lancia Stratos, an Alfa Romeo Montreal, a Mercedes 300SEL 6.3 a Jaguar XKE and several "lesser' vehicles, including my own 60-mile Corvette C8 and during the process, i found my 914-6GT.
All transactions originated by a request for a specific model by an interested buyer and were completed within certain "communities" - racing, clubs and loose affiliations of collectors. Still, the offerings outside auctions are just not there.