I have built my collection (a narrow field) mainly through dealers (a very narrow field.) They are also experts. They also have the goods. My knowledge is modest but my reference collection due to these dealers is so good I am able to correct Siegel,Kelleher,Aldrich & others without certs. I do purchase at auctions and
ebay. It is more problematic, but I have my references to compare. The dealers are retail but usually not much if anymore than the auction houses. The service, attention,knowledge & quality well worth the exchange.
My humble experience in the here and now. My collection now is the highlight of 61 years as a collector. Serious no. Investment no. Fun. Like a 12 year old.