The things to check are the issues that arise if the mileage is TOO low, are the fluid changes frequent enough, is all rubber still supple, car always been garaged, the parts well lubed, the AC cold, the soft top/rear window in good shape, everything works as it should, on any good car?

He claims the provenance is good - proper paper trail on maintenance- there is about a month of a Starmark warranty, from Beverly Hills Mercedes Benz,
the color is not offensive (white)

and given that the model is pretty scarce, and well known to be a very good bet since it is a W124, and probably it is virtually impossible to find with under 10,000 miles on the clock, if you are looking for a car like this, if it really is everything it is claimed to be, it is worth it to step up to the plate and go for it.

It would be like starting out with basically a brand new car, AND a brand new Mercedes cabrio with most of the modern amenities like ABS, SRS while STILL getting one of the last ones but to an engineering standard "Die best oder nicht" (The best or nothing - G. Daimler)

Fifteen years from now, (whoever buys it for $30k today) that person will
not be thinking that he paid too much, but, presuming the car has had proper care in the interim, be marveling what a beautiful and wonderful, classy, and likely safe and troublefree his car still is, and with years and miles still left in it to enjoy.

I would say the two bidders at $30,000 realize they will have to pay high at the beginning if they want the best example possible of a car truly worth having.

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)