The thing is, most of us who have Model 18's, don't seem to sell them.
+1 and IMHO the biggest factor in the recent price increases. My M18-2 is the only gun that accompanies me on
every range trip, and it's the last gun I would ever sell.
FWIW I gladly paid $500 for it ~2 years ago after scouring gun shops and shows for months. It was in 98% condition, it had obviously been fired but not very often, and it did not have a box or papers. The M18-2 is one of the less common variants. I could probably sell the gun today for the same amount I paid for it even though I've put roughly 4,000rds through it.
...in general most fairly priced M18s are gone within hours only ones that seem to linger are the +600 dollar ones.
Absolutely agreed; although prices have climbed, IMHO a M18 had better be truly LNIB collector-grade, preferably with a box and papers, to be legitimately worth much more than $600, especially if it's a M18-3 or later.
Example: One of my LGS's has a ~90-95% M18, no box or papers, on consignment for $700. It's been there for a while. It will probably be there for a while longer.