Well, why is her being a "biggun" a problem? A younger ball shouldn't act any different(except some very young babies can be nippy and nervous). It won't look much different, unless you buy a morph instead of a normal.
Your "younger" ball will grow up big too. Sometimes taking very little time to do so. A younger ball won't behave differently just because it's younger, and you don't particularly "teach" balls things.. so what exactly would you gain to trade in the snake you have for a different snake?
All that aside, if you find a good new owner, it's not going to matter to the snake if it's owned by someone that appreciates her size(like someone who might need a big female to breed) more. You might want to be certain of the sex before you put her up for sale, as many of the people I've dealt with selling females often turn out to have males. But a larger female would be fairly easy to place, normally.
Just wanting to make certain you realize what you want or will end up with might be the same as you have now. Wouldn't want you to go through selling this one and buying a new one, only to be dissappointed again.