The main purpose of a watch is to tell the time, right, so as part of the quality control we do, we check how well they do just that.
The accuracy of mechanical watches and particularly vintage mechanical watches are not to be compared with modern quartz watches or your mobile phone.
High-end mechanical watches, such as this COSC certified pre-owned Rolex Datejust, do keep time very well as long as they are serviced and regulated properly.
Vintage watches in the affordable price bracket, such as this vintage Smiths watch is naturally not going to perform as well as a modern Rolex watch. It is because the movement is of a simpler design. There are fewer friction points that are jewelled and the movements of the Smiths watches don't have an adjustment arm for example.
Wearing a vintage mid-market watch is not the same experience as wearing a high-end Chronometer certified watch like the Datejust. You are going to need to wind it more often and you are going to need to set it more often. But this is one of the many reasons why collecting watches of different types, ages, and price points is such a rewarding hobby. Each watch has its own personality and there is a time for every watch in your collection.
Now, with the above in mind, you will have realistic expectations regarding the accuracy of the watch you are considering to buy. But if you want to buy a pre-owned watch online, without seeing it in person first, one of the things you do want to know is how accurate it is.
For this reason we time all watches we sell. We use a timegrapher to test the watch is six different directions (dial up, dial down, crown right, crown up, crown left, and crown down). The reason for this is that the accuracy - even of a Chronometer certified watch - depends on the position of the watch. Gravity affects the escapement.
In addition to the accuracy, the timegrapher tells us other valuable information regarding the health of the watch. By inputting the lift angle of the movement, it measures and calculates the amplitude (how far the balance swings) and the beat error.
Some very expensive mechanical watches called tourbillon watches have the balance wheel and escapement mounted inside a rotating cage, so they can be regulated to a consistent accuracy regardless of their position.
Until we get a tourbillon in stock we'll continue to test our watches in six directions.