Should all the door knobs be the same for the entire house?

I tend to agree with your thinking to paint them but realtors know their business and the market. However... I would not change the knobs.

nice older wood doors with brass or even glass knobs are huge selling point(don't know if that is what they are) anyway listen to realtor save your money for other expenses that will bring more return

Much depends on the age of your doors and knobs. If this is a really old home, like 100 years or so and the door knobs are all metal you can remove them and spray paint them black if they are decorative or glass leave them as the are. Same with the doors, if they have never been painted leave them. If they were painted you may need to cover old possibly lead based paint, so paint. If this is a house of the 70's with slab doors and basic door knobs you should do what you can to update the look.

yes. its part of the "charm" of your house. the second answer is a new knob may not fit the door. older doors seem to be thinner than the newer doors. I've had to sus out replacement doors for my 50's fixer from a habitat for humanity restore.

If it is a vintage home, listen to your realtor. People spend buckets of money unnecessarily and then new owners come in and replace everything to suit their taste. Save your money for your new home.