Certainly buyer beware is standard advice, but the idea that "serviced" doesn't mean "fully and properly serviced" is very odd. Not sure why you think this term is so vague, as there really is no debate regarding what a service is.
A service includes a full disassembly of the movement, cleaning, inspection of parts, replacement or repair of parts as needed, assembly, lubrication, and adjustment/regulation to ensure that the timing is within specs.
If all that is not done, it's not a service.
Omega would refer to anything less than this as a "partial service."
The issues of what's done on the external parts of the watch is a different story, since there are concerns related to originality, patina, etc. for vintage watches, but there's no uncertainty about what service means for the movement.
There are certainly plenty of watchmakers out there who don't do a service properly, but that doesn't mean that the definition of a service is somehow not defined.
As many have said here, unless you have documentation that proves that a watch was serviced by a known good service provider, then factoring the cost of a service into the pricing is a good idea. If it ends up not needing a service, then consider it a bonus.
Cheers, Al