I had corresponded with you about this car because it does fall into a rare category. For me, it is a convertible capable of seating five. The only convertible that seats five made since the 70's is a hideous Oldsmobile Cutlass that ended production in the early-to-mid 90's. It is no comparison to a Benz.
I wanted something sporty, but started considering the options of a bigger car, but one of the killers for me, since I like to work on my own cars is the amount of space that the larger cars would take in the garage. I would not be able to walk around the front or back of some larger cars, at least not with the garage door closed, which is really important in the winter. So I ended up eliminating the big cars from the list: Impalas, Continentals, Delta88's. Not to mention, being an r107 owner, I've gotten to really like having a "sporty" convertible.
So, when I was shopping for a 5-seat convertible, I had decided to go with what was the most common, but still "sporty", a 66-72 GM A-body (Cutlass/442, Lemans/GTO, Skylark, etc.). Those cost an arms and a leg in good condition, like $30k as a starting point for a good example. Sure there are rusty ones to be had in the $8000 range, but I want a car that can also be enjoyed.
I am a 107 nut, and fortunately, I came across this
. $10,000 should get it in "good nik" for me. (Maybe there will be a few more $1000 projects that fall into maintenance, but I can't tell if your car has the same issues anyway.) I wasn't able to find any flaws in the car, other than one major one, the radiator support being replaced. (Why else would someone repaint a car and make a chop-top out of it?) It is a total color-change, interior change, and obviously a major alteration chop-top. Those things probably eliminate it from the "collector" category it could have once been in, since it is one of the ~2700 Mercedes rally car homologation 5.0 liter 450slc cars (107026). A very good condition 026 will bring at least $7000, or up to $15,000 in some cases. Sure, there are folks still finding rusty ones, and ones that need work for $400 and $2000 (just in the last month), but in those cases the seller was not aware of what they had. (In the case of the $400 one, the seller was awaiting the crusher to come get the car for $300 because the seller didn't want to fix the windshield wipers and the front bumper was wrecked.) So, maybe the market for the 026 is similar to that of the 560sel... if it is reasonable that someone would pay $15,000 for a nice example.
It is my opinion that the 107 has more classic styling, with a body that was designed in ~1971, whereas the w126 is much newer and less of a classic. So the w126 in my opinion is still falling in prices, whereas the 107's are rock bottom right now, and hopefully starting to go up.
So my analysis of where the value falls on this w126? Probably right about where your offer was. Hold out for over $10,000 in my opinion, if not even $15,000. That ebay car looks like it could use up to $10,000 in work to get it back to where yours is anyway.
Oh.. last comment... Get rid of that hideous grill! (Sorry).