WTB late 80's XJS

As with any older car, value is often in the eye of the beholder, but to get a general idea, you might check https://www.edmunds.com They only go back to 90 on the used car values, but this should get you in the ball park.
PS: Welcome to the forum!
Basil

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1967 Jaguar E-Type OTS
1E13333, Silver Blue

What things should I focus on in inspecting/setting value on perspective cars?

TIA

Late 80s?

Well, by then most of the rust problems will have been cured.

On an earlier car it is essentially to check everywhere with a magnet no matter how good the car looks externally, because MK 1 XJ-S' rot like there's no tomorrow.

I'd take the car to a specialist. That V12 is one of the most complex powerplants available in a non-exotic of that era (although some would consider the XJ-S to be an exotic). Most people get scared off by the complexity so make sure it's been serviced well.

Make sure this specialist looks over the electrics and the rear end, because these are problem areas.

Good hunting!