It can be as simple or as complicated as they want it to be. Most of the time the IP is leased to somebody else within a limited capacity of some kind (e.g. for a certain duration, with certain restrictions, etc.) and then rights are returned to the owner. If the licenser does a bad job, the owner just needs to wait for the rights to return and license them to somebody else to try again.
Selling IP is permanent and irreversible, which is why it's not a very common occurrence. Most of the time, IP sales are the last resort of a company clinging to survival. It's actually often more common for the buyer to buy the entire owning company outright in order to acquire the IP than to buy just the IP specifically. Microsoft bought Mojang for Minecraft and Bungie for Halo, and Tencent bought Riot for League of Legends.
The last time we had a big IP sale like that without the seller facing massive financial hardship was probably when Square-Enix decided to divest from most of their western dev studios and sold much of their former Eidos holdings to Embracer group, specifically Tomb Raider, Thief, Deus Ex, etc. along with the studios that developed those franchises.
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