Chris,
Sad news - if you do sell
From my perspective, there are a lot of pre-owned MINIs on the market this year - the '05s triggered many upgrades, most especially from owners of the quicker MINIs. Presumably due to the LSD option among other things.
I don't think KBB and the others have caught up with the "saturation" of used MINIs yet.
Last I saw,
GBMINI#2 was at $23.5K. That is for an '03 about 10 months older than yours, and with 28K miles - similar.
What is next is TOTAL "finger in the air" - I don't check used prices of MINIs regularly and I am not in contact with MINI Peabody all the time (no matter what people claim

)
If your car was about $25K new, I think you can get $20K for it now.
The
JCW upgrade does not seem to be offering the investment that we hoped it would, so you might get $2K extra for it.
So I am suggesting you can sell at $22K - you advertise at $23999 and cross fingers.
Now I am well known to be an extreme pessimist.
But I also know of some MINIs owners that are having trouble selling their old one - and of course there is GBMINI. Some of the slow sale there might be due to customisation, but MINI Peabody has had other upgraded MINIs on their lot that have not sold for what you would expect.