To be fair, the buyer could have asked all the questions they needed to before purchase (if not in the listing), then found an issue Only upon opening it. Which isn't recommended except for a professional.

If I sold a watch here, I wouldn't be opening it to answer any questions if it was running well and in apparent top-notch condition. It would be sold as is, in running condition, with a lot of photos. But not the insides unless I was a watchmaker or knowledgable jeweller (which I ain't).

As a buyer if I received a watch in good running condition I wouldn't go and get it checked either, right on receipt...if it faltered or failed I would. After giving it a few days to settle in if running but not keeping good time at first. (See my post above)

Opening an expensive watch means replacing the seals that make it water-resistant/proof depending on what it is rated as, for one thing. 

Buying an expensive used watch does mean research...prices, and what movement is has if buying as a collector/investor. 400$ isn't too pricey for a ten year old TH in good condition if it's the model or look you want. Again, depends on the model. Can vary a lot for the same brand.