The seller has had this listed a couple times on eBay and it hasn’t sold. It is listed as a 1976, but while that may be how it was registered I’m certain this car was converted in the early 1980s.
It is riding on the W126 500SEC based chassis, not as the seller states the 116. The engine is the 6.9 liter out of the 116 based 450SEL 6.9, though. (edit: This model is on the W116 chassis, unlike the 5.0 liter versions that came on the W126.)
While this one is an automatic, these cars were available with a 5 speed manual and with a bi-turbo engine 5.0. When new these cars started at 250,000 German marks.
The Swiss based Franco Sbarro company still does business on high end vehicles.
The seller might want to go with more classic rims and include more information and pictures of the engine bay if they want to get anything close to the $90,000 ask price.
At first look this indeed looks like a W126 chassis vehicle. This has the W116 dash, something I easily recognize from my days of 6.9 ownership, and distinctive U.S. market 116 servo climate control center console, something my 6.9 didn’t have being grey market.
Two sources I used in digging up some info, Customizing and Tuning Mercedes by Heitz and Neff and Extraordinary Automobiles by Vann and Asaria both list Sbarro as using 500SEC W126 chassis as the basis for these conversions, calling it first the Shahin 1000 and then the 500 Portes Papillon Gullwing. It turns out they did use 116 chassis parts when using the 6.9 liter engine as opposed to the 5.0 or the 5.0 twin turbo option. Apparently the 6.9 motor was not available with the twin turbo option. It was one or the other, though who knows, money could lead companies like this to do strange things for customers.
There is also some wonderful additional photos and information available over at 1000sel.com and this Sbarro Shahin website which quotes directly from the Vann book and uses the same photos from that book.