I am not saying overpowered in the means of breaking the game but just what he intents to do with it...
He wants to sell castles and the listed price in DMG is 200000 gp this is what I find Disturbing...
Of course in world where you can abuse wish this may not be such a big deal.....
If you are the DM, feel free to turn the process into an adventure! Not-so-random encounters can find them (bandits, slavers, assassins, competition, etc). The tons of spell slots needed should take quite a while (depending on their character level). If you hand-waive the time away, then that's your fault. Every day could bring untold adventure to them.
Another take on this using the wealth-by-level guidelines: let them make cash hand-over-fist playing monopoly... but oddly enough, most combat encounters afterwards are animals, plants, penniless humanoids, vow of poverty folks, incarnates, pretty much zero loot encounters.
Also, you can feel free to adjust the selling price all you want. They call it the Dungeon Master's Guide for a reason... That's your resource, not his! (where is that castle reference btw?)
Your player may control his character's actions, but you control the actions of every NPC in your game world. Just because there might be an NPC willing to sell a castle to the PCs for 200,000gp, that doesn't mean that the PCs would be as sucessful finding an NPC willing to buy one for that much.
Let a potential buyer string them along, but he complains about architecture and wants everything customized, but is still never happy. Suddenly, the buyer mysteriously disspears!! Do the PCs investigate? Do they search for the lost buyer... do they adventure?!