They should also study the market place. Even with Patek Philippe, there are certain models that don't do very well. Check the condition. Refurbished watches are not as valuable in the collectors market. It is better to have a watch that is slightly discolored or a little dirty.
Some people feel they just want to buy as many watches as possible.
They don't necessarily look at the actual quality. Condition makes a big difference in the price. On the whole it is much cheaper than paying a dealer at retail. The movement should be serviced once every two years or so. Take it to the manufacturer or someone reputable.