If I buy an entire row of seats on a long flight so no one is next to me, can the airline sell one of my empty seats to someone or will they respect the fact that the empty seats are mine to do as I please with?

It’s weird that communications doesn’t work both ways, but unfortunately, people have to be checked in. Once the airline realises that those seats aren’t going to be filled; they’ll try to sell them to someone else; providing that there’s no-one waiting on stand-by. If you check in at the desk, someone may ask where the rest of your party is. You’ve got to make an arrangement with the airline first if you need more room. It’s difficult because they want to out everyone into the same bubble which makes things easier for them to deal with. But companies are different. You need to check the booking terms and conditions when you book. And if there’s a good deal, they need to put procedures into  place so that their competitors don’t buy these seats which would force customers to seek out more expensive flights. Know your rights before you go. It’s difficult to look them up once you’ve boarded!