If you buy the same brand and they're not that many years apart in age, it should be possible to rekey to the same key. In fact, I've even seen kits in hardware stores that let you do it yourself.

For the time the house is on the market, why not just use two different keys? The new owners will want to rekey the locks when they take possession.

I would agree with Gray, first weekend I had my house we rekeyed the locks. 1) to make them the same, 2) to lockout all the extra keys that were floating around.

And yes you can get them keyed the same. I'm not sure what they need. I think maybe just the new lock and old key.

I have a related question -- our front door has a separate lock and door handle. We replaced the lock a couple years ago with a keyless model and now the the door handle part needs to be replaced. Where can you buy just a door handle?? I don't want to pay extra $$ for the lock when i don't need it.

"Where can you buy just a door handle?? I don't want to pay extra $$ for the lock when i don't need it."

Its called a passage set and is normally only used on interior doors.
They are rarely good enough for use in an exterior door and exposure to the weather.