Why would someone pay so much?

There are a couple of ways to look at that plane and it’s sale price.. First- you can purchase multiple used Control Line models at bargain basement prices. I’ve bought 2 for $5, Kim got a 5 gallon pail full for cheap at a Swap Meet, if you want to buy cheap, you shop till you find a bargain.
Second- maybe the winning bidder had a similar plane earlier in their life, and wants to recapture that moment or memory of excitement of flying with out spending the time required to buy a kit or ARF, build and finish, buy and install the engine,tank and controls.
Let’s add it up , rough estimates.  Ringmaster kit $75-90. + finishing materials $30 + motor &tank $50 so, $120 to $150 in supplies. Labor? How long does it take to build a good straight airframe? 6, 8, 12 hours or more? I know my leisure time is worth more per hour than my paid working rate, but let’s use something above minimum wage, a little higher that the starting wage at Mickey D’s and assume your a speed builder like Mark!  10 hours at $15 hr is cheap at $150 labor = +/- $300.00. I don’t think he paid to much for something he wanted. May not be a bargain but a fair price between a willing seller and willing buyer.
Ebay gives a much larger audience than a local clubs Swap Meet or regional postings on Craigslist so the chances of finding a deep pocketed buyer is more likely, but the fees are still covered in the shipping cost and higher listed prices.

Agreed, time to build is a rare thing for many people.