If a watch has been serviced, it has been taken apart, cleaned, any worn or damaged parts replaced, reassembled with correct lubrication and tested afterwards to ensure both timing and auto winding is functioning as expected. Someone like Archer could explain that in far better detail but as a novice collector the above is what I typically expect from a service.
Sometimes a seller will take a watch and regulate it, or adjust the timing to make it accurately keep time even though its in a bad state. That's a $25-50 job.
Sometimes a seller will take a watch and clean and lubricate it but will not replace damaged or worn parts as it would add to the expense too much, that's not doing it properly. There are many degrees of half-arsed they can potentially do that are "good enough for sale" solutions.
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