Right now I have 5 bikes, I know for some of you that is not much. 4 steel, 1 Aluminum.
1973 Raleigh Suoercourse - Commuter
1981 Bianchi Limited - Brifters added, comfy
1984 Torpado Super Strada - Turned into Townie (never fit right but I love the looks of it)
2001 LeMond Nevada City - one of my main rides
2013 Felt Z85 - Main Ride (Aluminum)

But my question is this:
There is a 2008 Specialized Tarmac Elite frame for sale here, in VG condition, for $300. It is the right size and the right color (blue). I have never ridden CF. I don't know if I'd like it or not. Not sure how CF holds up over time. Wondering if it would be money well spent? Would I gain anything? Or am I just drawn by the bright light?

What would you do?

Unless you have all the parts to build it up, I think you'll be upside down on it. For the money that its going to cost to build you could buy a much better bike.

Is it the frame/fork set - or just the frame --- if I were curious and had the resources to build it properly, a complete F/F for $300 would be tempting ---- but if its just a frame only - i'd pass.
It would almost make me suspicious it had been curbed hard and parted out

The Elite now is considered one of their entry level models (Around $2400 complete) - but I don't know the difference between the modern version and one from 8 years ago--- unless you had a ton of late model parts floating about - i'd figure on another $1000 to get it built -- and that's factoring in something like a Shimano 105 group from the UK and a cheaper set of wheels to get it rolling

-- which begs the question --- how far would 13-1500$ go towards one that is already complete and possibly a little newer from an ebay seller



I will admit I took my '13 Cannondale carbon fiber bike out today ----- I was originally going to take out my late 80's DeBErnardi , but I gotta admit the compact drivetrain won out --- (was windy - didn't want to push a 53/39)

Its almost not fair to compare a modern machine that is tuned properly to my vintage bikes for ease of use. Riding C&V is an experience , - kind of like driving a car with a stick shift --- but the new one just disappears underneath me and is not noticed because it it never does anything dramatic - just bland and boring -- but my average speeds for my outings on it are usually quite good

The Tarmac is supposed to be more responsive (read 'twitchy') than the Roubaix. So depending on the intended use, it might not be what you need.
The original specs are here Road Bike Action | Specialized Tarmac Elite
and a more sober review Specialized Tarmac Elite review - BikeRadar Australia

but would I buy an 8 year old carbon frame? No.

My CF build is somewhat non-stock. So, I'd rather build frame up, rather than replacing various components with what I'd rather have on the bike.

$300 isn't bad for a complete frame (and headset?).

It is Frame, Original Fork, and additional fork, headset, Ritchie stem and seat post.
Apparently the original fork had been cut down for zero stack height and then the guy had back problems...so bought a new one with longer steerer. I have a set of wheels, saddle, and bars that could go on it.

I guess it's really the unknown of 8yo CF that scares me. I'll probably pass.

This is why I like threaded headsets.

My wife has a Tarmac Elite and it's a nice bike. There are different grades of carbon now which I don't expend any effort to try to understand, but I think the Elite is maybe one level lower than the S-Works stuff, then sold with mid range group (although my wife's has DA upgrade). If I had your stable I would look at it as replacement for the Felt Aluminum. I don't have any real qualms about used CF, the Tarmac was bought used and I have 2 CF Colnagos that Ive been riding for 10+ years that I bought used.

If I were building a carbon fiber bike to do any degree of hammering, I wouldn't go as old as 2008. The technology continues to advance and a 2008 model is just obsolete.

If you're determined to get it, you'll want to check for signs of fatigue or delamination. My Tarmac is a 2011 and it is starting to show its age. Years of hard riding and flexing where the aluminum dropouts and bottom bracket join the carbon have caused all the paint to come off in that area. I check it regularly to make sure the joint isn't actually cracked. So far so good.

As far as the Elite model goes, I think think the difference over the Comp was in the components. I don't think the frame is upgraded until the s-works level. Could be wrong on this.

Carbon fiber has a good track record in aviation. The airplane I fly has quite a lot of carbon fiber, especially in the tail. The oldest one at the company is 6 years old and has 26000 flight hours on it. I don't worry about the plastic parts at all.
However, all of my bikes are steel. I really don't like the harsh ride that I've found on the plastic bikes I've ridden. Life's too short for a crappy ride. For that reason, I'd pass on the frame the OP mentions.
Look for a used modern steel frame. It'll be harder to find but much more enjoyable in the long run.

ITs not too bad

there is a "vintage carbon" thread in here somewhere showing bikes still actively is use from the early carbon days (early 80's bonded carbon/aluminum )

now that you added that 2 forks are included -- it sounds like a solid deal if you wanted a project like that