MacBook Pro resale value -- should I replace the battery?
I think if the rest of the laptop is still in good condition, then replace it.

For one thing, it leaves a bit of a bad impression with a potential buyer. You're giving them a machine which works only with the plug and if they wants to use it on battery they'll have to go out and work it out with Apple himself. Personally I'd expect a pretty hefty price reduction for that.

If you replace it on the other hand, you can clearly state so in the description (eg "battery was recently replaced for $120 and should now allow you to work for a full X hours while not plugged in" or whatever) which actually increases the price and the value of the laptop.