I think it's quite the opposite. Unless you are picking up ultra collectible gens, your regular bread-and-butter gens (Omega, Breitling...) will lose you more money than a rep. In fact I have several rep builds which are going up in value. Take an FGD dialed high-end Pre-V built as an example. Would you have expected to fetch well over $1,000 without any gen parts a year or two ago ?
In addition modding your rep is fun and likely to ADD value, instead to depreciate your gen even further...
I am just selling my last gen because to be honest it has not seen any wrist-time in 2 years.