It sounds like their modus operandi is to offer a good price sight unseen to make sure you turn up with the watch for inspection. Then when you turn up the watch is found to be sadly deficient, in need of a service, etc. and they throw you a low ball offer, take it or leave it. I suppose enough people take it to keep their business blooming, they seem to have a ton of staff including a gaggle of watch polishers, etc...
On the surface this seems like a legit way to do business, as long as they are honest about the watches needing service. However, saying that a watch needs a service when in fact it does not seems to amount to fraud, it is at the very least very dishonest. I would get a second opinion.