Everybody's favourite auction site does tend to over-use the word 'rare', if you were to go by what you read there, 99% of all mechanical watches are 'Rare', or 'Unique.'
Unique means one of a kind, and obviously, should only be used when describing exactly that.
So what about 'Rare?'
Most companies that produced mechanical watches in the last 150 years made a LOT of them, sometimes millions per year. Depending on what country you are reading this in, what we see everyday may differ from what is common where you are. Here in North America, we didn't see a lot of Cortebert or West End, but those companies made a lot of watches. In Britain you saw a lot of Smiths watches, and I see that the prices for these are rising steadily, eclipsing most Omega vintage watches.
Which brings me to another point.
Don't accept as gospel the inflated prices you might see online. There is a slump in antique clocks right now, and we stay in touch with many US dealers as well, who report the same thing. Pocket watches have taken a real nose dive as well, and in general the Industry is experiencing a lull. I don't know about the brands themselves and their new product, although certain well established makers seem to have no issues.
There is a lot of competition out there, and small companies pop up all the time, kickstarting their way into the market. If you look at the specs of most of these new mechanical offerings, you will see the same old ETA movts or their Sellita equivalents, followed by Citizen driven watches, usually the 9015.
(There has been a marked drop in the quality of some ETA movts, and in the case of the Sellita SW300, which is a version of the 2892 ETA, improvements have been made.)
Until other manufactures up their production to compete with ETA, unfortunately we will continue to see a lot of this.
There are still many gems to be had, watches that are under the radar, and when these start to get their just vintage due, the prices will rise. Watch manufacturing history is pretty convoluted, but it is well worth the time to find out about makers that either didn't advertise all that well or who are now largely forgotten. The Grand Daddy of them all is Eterna, watch that one.
So definitely read the specs, (I'm talking new watches mostly here) and don't believe everything you hear or see, especially on Auction sites. That word 'Rare' is pretty common, and that mass-produced brand name watch might not be all it's cracked up to be.