es1bkacsur

Like get a better idea of what it is & what it's worth. No model name or number on it, just Cannondale but it matches up to the pics & description of the SM600 in the 1987 catalog. Serial #18 082686 024 I think says it's an 18" frame made Aug 28 1986. The seat is not original (who puts a Bontrager seat on a Cannondale). Handlebar should be "7000 Series Alloy" according to the catalog but a magnet will stick to it so also not original. Best I can tell all else is original including the tires. It is the model with roller brakes, 24" rear wheel & 26" front wheel, maybe something called "Mombat"?
As to worth, my original impression was with plastic pedals & hubs not quick release only worth $150 maybe $200 in my area. Checking the usual suspects on-line (eBay, etc.) I found these listed from $300 to $555, bare frames $200 to $400 & some website called budgetbicyclectr.com asking $1399 so I figured I should get your opinions before I undersell myself. Any info the bike and/or value greatly appreciated.

es1bkacsur

130 views & no replies, is this really that rare?

Pantah

It's just that there's nothing particularly special about it. The whole 24" rear wheel thing didn't last long, not especially popular. Components are decent enough quality but nothing to write home about. Cannondale makes a quality frame, I'm personally a fan and have four in the garage, but a mid to late 80's Cannondale mid-spec mountain bike just isn't worth much. There are older models worth something but yours ain't it.
It'll be a fun project if you want to clean it and tune it up, but even afterwards you'd be lucky to pocket $200 from it.

SoCaled 

Ebay listings are only useful if you are looking at Sold/Closed listings. Budget bicycle center is a bit of an oxymoron, they famously have a Huffy Aerowind listed at $1,999 when they are listed on Craigslist, etc @ $10 - $100, not the place to find "market value". Agree with you and above posters, probably not a very high price even if cleaned up. Price will depend on your market, patience and how well you clean it up and present it. I have seen these sell at the $300-555 you quote above, but mostly pre-Vid. I have posted them (or similar) in are you looking at $75 - $200. I like to keep working bikes together, but you might consider parting this one. Roller cams, Deore group, and frame, individually on ebay, but that is time and effort.

Pantah

Above comment is definitely sound advice.

In an attempt to give you a real number for the bike you actually have, I'll say what I would pay were you selling it to me. In the current condition, exactly as it sits, I'd give you $100. If it were fully cleaned up, tuned up, new tires and tubes, (original cables and housing would be fine long as they aren't rusty and/or frayed), lube where it's needed, repacked bottom bracket, headset, etc., I'd give you $200, maybe $250.
Hopefully that helps.

VegasTriker

There is a handy site that allows you to determine the year it was made by the serial number and has catalogs for the vintage you are looking at. The color doesn't match the 1987 SM 600 which is shown as light green. The yellow color was used in 1985 for the SM600 as one example. I did not know about the site but dated my Cannondale SM800 (which I bought for $20) by the code on the inside of the Shimano Crank. The crank has a two letter code that is the year the crank was made which is likely the same year the bike was made.
website that shows where the serial number is and how to interpret it https://vintagecannondale.com/info/serial_numbers/ and https://vintagecannondale.com/catalog/
The 1985 Cannondale catalog from the same site https://vintagecannondale.com/year/1985/1985.pdf