I used to be a major collector of lobby cards from all mystery/thriller/detective/spy series films (including Chans, which were my favourites) and had several thousand cards overall that I amassed over about 25 years. I still have a few Chan cards I keep for old times sake in what I am calling my "permanent collection" but I have auctioned off many of my cards or sold cards to dealers and other collectors that I know privately or generally via ebay.
I sold and continue to sell cards via consignment to Bruce Hershenson's online site at emovieposter.com. I have always found him the most trustworthy auction outlet with accurate descriptions, great packing, etc. Some of the Olands I consigned, notably title cards and cards from the lost films, made what must be considered record prices. I thought I had overpaid for those via major auction houses, dealers, ebay sellers, etc. (I have never found any in antique malls, local collectibles shows and such although I keep trying even now) but the prices many sell for now are astounding -- I tell people if they were that expensive when I collected I would never have been able to buy any! On the other hand, recently I have seen several cards listed in Bruce's and various other auctions from the later films, notably the Monograms, and the prices seem to be coming down on those...I guess many collectors are getting older and realizing it is time to sell.
Old habits do die hard and depending upon price, I will buy the odd card for my permanent collection, notably ones I have looked for and usually never seen in my many years of collecting. I would love to find any card from The Case of the Howling Dog (Perry Mason Series) or Murder on the Blackboard (Hildegarde Withers series), two films for which I have never seen a lobby card and indeed, no posters other than I think one mini-lobby card for Howling Dog. I might be tempted to bid by a decent Chan Black Camel card as I have always been outbid -- Black Camel cards are also rare but they are not the rarest Fox Chan title -- I think Charlie Chan's Greatest Case cards are the rarest of all Oland titles.