I just saw an ad on Facebook:
(Translation and bolding mine.) I can’t imagine a situation where someone has a 2006 Mustang GT convertible and just happens to be in the market for a 2014 Spyder (sort of a 3-wheeled motorcycle).
I’ve been seeing ads like this for decades. Sometimes it’s completely unrelated, like a faux leather recliner offered for sale, willing to trade for a gas barbecue grill.
Have you ever seen a situation where this kind of trade actually worked?
I had a relative respond to ad where guy wanted to trade a used pick-up truck for a used pop-up camper or travel trailer. They actually went through the trade and far as I know both were satisfied. However, the trade you are describing is very specific; I doubt he will have many(if any) responses.
“Take imme-di-ate delivery on this '57 Chevrolet half-ton pickup truck. Will sell or swap for a hide-a-bed and thirty-five bucks. Call One-four-oh, ring two, and ask for Bob.” 
“Took two full quarts of 40-weight o’l just to get 'er to the Conoco station.”
In the custom / highly modded car world, I’ve seen this happen successfully plenty of times. Trades can be between two folk with custom vehicles, or between one side custom and one side normal.
In this case, it may be aided by the fact that it’s harder to value cars with a lot of custom work, and a lot of people put their vehicles up for ridiculous prices - if they then have a “will trade for a 5 year old truck” in the ad against the ridiculous price, truck-guy may think he’s getting a bargain.
It sounds like simply trading one nonessential adult toy for another. I had a Jeep Wrangler once, that was a second “fun” car, not my daily driver. After the second or third time fighting with the manual convertible top on the side of the road during a downpour, I decided I wanted a convertible Mustang GT instead.
I went on CL that night and lo and behold, there was a convertible Mustang GT : For sale $XXX or trade for 2002+ Jeep Wrangler! I contacted the seller, and we made the deal the following weekend. In my case, both the vehicles were worth about the same, so no cash went in either direction. We wrote the bill of sale up to reflect this, and as a result neither of us had to pay sales tax on the transaction. I’m not sure if that is treated the same in all states. I had the car several years, and was very satisfied with the trade. Of course, I had other Mustangs in the past, so I knew was I was getting in the trade.
As I was typing this, I recalled a few years ago, I was selling an old car. I always proclaim “for sale or trade” when selling something, because someone out there might offer me something I don’t yet know I want. A guy asked if I’d be willing to take a snowmobile for it. Now, I haven’t owned a snowmobile or really thought about snowmobiling in 25 years, but I wound up doing the trade. I’ve had a great time with it, the past two winters!