He's not selling it. he is either junking it or giving it away, usually to one of his wife's students.
When he buys something new now, I have started asking him about how long he thinks it will last so I can expense at least part of it via 179. Graphic designers have to stay really up to date on hardware; no computer lasts five years with that kind of use.
Apparently that's the correct way, although it shows up on Form 4797. If depreciation could be continued, that would appear on Schedule C and reduce SE tax. So using Sec 179 and other accelerated depreciation methods is usually the best choice -- or using a business entity that does not file Schedule C.