What do I do with the castles that can no longer be merged?

Sandcastles? If you have 2 or more and can spare the space, save them and someone will pop up asking for them. Good way to get extra coins but I’ve found it won’t work if you have 9 or 10 quests active (I have 10 people unlocked, saw somewhere there are more). If I close and reopen the game it triggers sandcastle or fancy cutlery quest if I have 2 or more on the board. Not sure if that will always happen though. Sry for long answer.

I currently have 4 max level sandcastles and no one is buying. Usually the buyer pops up at 3 castles. I’ve served others characters three times to open the space, but no luck. Something has changed.