Do companies/governments ever sell satellites that are already in space to other companies/governments for use? I'm wondering about the sale of the actual physical satellites. Has it ever happened? Is it at all common?
Yes, it has happened. This news article shows an example: Urthecast Buying Deimos’ Imaging Division and its 2 Satellites
Deimos Space of Spain and Urthecast Corp. of Canada announced a strategic partnership in which Urthecast will purchase Deimos Space’s fast-growing Deimos Imaging division and its two satellites, Deimos-1 and Deimos-2 — both in orbit — for 74.2 million euros ($84.5 million), or about twice the division’s forecasted 2015 revenue.
I don't think this is particularly common, but neither is it the only time this has happened.
It seems @djr's statement is correct.
A similar thing happened in 2008. AMC-14 failed to achieve geostationary orbit, and was left in a highly elliptical and inclined orbit. After insurance issues were addressed (with an interesting subtext involving orbital maneuver patents note: needs answer!) the US government purchased the satellite, and have used its thrusters to achieve an inclined Geosynchronous, but not Geostationary (GEO) orbit.
Space News: Insurers Sell AMC-14 Satellite to Pentagon
Gunter's Space Page: AMC-14
Defense Aerospace: SES Americom Declares AMC-14 Satellite a Total Loss
Some of the companies owning the satellites either merge or sell out to other companies, like PanAmSat, sold to Hughes Electronics, News Corp, then a private consortium, and finally Intelsat.
Then you have Iridium, where the company went bankrupt and a new company bought them.