I have had my MSP for sale for about four months, I have had TWO people call me on it. ONE of them came to look at it. But I have heard of people selling theirs very quickly, maybe San Diego just is not a good market.
I also thought about selling mine back to the dealer. I went in with 12,000 miles on it and they said that they would give me $14,800 for the car. I paid $23,495.00 for it, so I thought that a $8,695.00 loss was a little too much (considering that I have only owned the car for eight months), thus I still own the car (and to a few peoples surprise; ie t3ase, I'm starting to like my car once again). No flame intended.
So what it comes down to is how bad you want that STi. I would say go in knowing that you should get "at least" $16,000 plus for your car. If you can get that, I would go for it!
By the way, I bought my MSP out-right.