I put together my first Mongolian collection in the late 90's. Material was scarce but I managed a pretty decent early collection that I sold for a song in 2007. Prices were all over the place then too--I think I picked up a counterfeit set of overprints for about $10 and and one of the $1 overprints with a certificate for $150--SCV $450. 32-44 (the ying yangs) SCV were about $1 each--I think I found 4 total for sale. This was in the 90's when there were still a half dozen stamp stores in the Boston area plus a show every weekend. eBay
had about 3000 listings for Mongolian stamps I swear I looked at every one of them. Lots of CTOs, silver and gold foils from 1995, and lots of topicals.
China probably has a few collectors of Mongolia too, there are more Mongolian people living in China than in Mongolia. I believe last year's Mongolia explosion was most likely fueled by China.
I'm always looking to increase my Mongolia holdings. Earlier in the thread I mentioned Dr Orth as someone whose certificate I would trust. There are very few certificates out there and I would trust the old ones over any more recent ones. There are some great articles in the Journal of Chinese Philately around. I used to have a folder of articles but I think when I sold off the collection I must have included them because I can't find them. Stamp forgeries has a basic primer on the overprints.
Earlier in the thread Randy pointed out that Kelleher (I think it's Kelleher) has some material in their next sale and the overprints are fakes.
I got my 74s from Ross Wiessman. He reported getting them from a Mongolian dealer who came to the US selling stamps. I think it's from the same holding as one that wa auctioned off a few years ago--I want to say at Siegel but I can't remember for sure. I'll see if I can find it online.