Flipping or personal use?
Rarity (and hence market value) for fans depends alot on many different factors - there is no easy rule of thumb. Some, like bank teller fans which blow at face height for use in banks and places with paper, command a pretty penny because not many were produced. Others, like fans from Victor (of record fame) are also worth a couple bucks because of the odd name association - during the 1920's and 1930's being a fan manufacturer was a bit like the dot-com boom: everyone had to have a piece of the action including firms which had no reason to make fans.
You can get a up to a grand or more for a solid example of one of those.
What I look for when I buy for personal use are:
Brass, paint and weight.
- More brass the better. Brass blades are a very very very good sign. Brass manufactures plates are also good. But brass has always been more expensive than iron, so the more brass the more expensive. And every part of a fan has once been made in brass - from the base to the grills - so its not just the blades you need to look for. These are easier to deal with if you don't mind a toothbrush and bar-keepers friend for 20 hours of your life after you find it because you need not worry about...
- Paint. Your not going to flip anything with a bad paint job. Just like furniture. If its MCM and Teak but trashed its not worth as much. Paint on metal is much harder to do right and evenly than refinishing.
- Weight. Old fans (and also blenders up until the 70's) have big motors. I have an old 1960's Oster Blender has a copper windings the size of a newborn and must weigh around 10-15lbs. Compare that to a current production Oster at Walmart. Fans have bigger motors. Then there are all the cast-iron parts. More cast iron the heavier. Really heavy usually in my book means older because of those two factors and usually alot of brass. That fan I showed takes two hands to move for only being about 18" tall.
There are some features which help determine age, like the type of oscillating design (if it has one), etc. But I don't know enough to really get into that.
Unless the fan is really unique, I buy whatever I can find at $30 and below as long as its an AC unit. There are DC fans floating around (very early fans) so read the plates if you want to use it. For example, that fan I posted was only $10 and assuming I don't need the blades balanced, the magnets re-magnetized (happens in high enough heat), re-wind the motor or decide to bead-blast it for that perfect (abet devaluing) finish I'm expecting to drop about $40 in small parts (period reproduction wiring, etc) and 8 hours active time to clean it to my standards. Bead blasting and I could easily go over $150 and 20 hours. Rewinding, 30 hours and alot of money in proper gauge magnet wire (usually the thick gauge, expensive stuff). Ergo, I would pay $50-70 for a great condition fan that just needed the oil trays or wicks cleaned. I've sold a few but only when its a larger project (one 24" cast iron, 50lb monster I again got for $10 with a trashed finish comes to mind) than what I intended to chew to people who restore them and I push the upper limit (~ $50) of what a restorer grudgingly accepts.
I know this may not help much, not nearly as easy to determine value as with clothing for flipping, but if you have an smart phone use that.
I've got a Sonim which gets dumber and gains character every time it gets chucked it out a third story window. It's no help.
That said, I find it fun walking into a Restoration Hardware looking at some over-priced made-in-china nickle-plated paperweight knowing I have the real deal at home with a motor so large and devoid of safety features it could double as a meat slicer in a pinch.