Rig at stock clockspeed does 1100-1200 M/hash on deepbit
Um, the screenshot you're showing has the rig outputting less than 1000 MHash/s total. Yes, every so often it will get lucky and temporarily display a speed of 1100-1200 MHash/s on Deepbit. By the same token, every so often it will be
unlucky, and will display a speed of 700-900 on Deepbit. You need to take the all-time average, not just the highs.
PSU:
Cooler Master Silent Pro M100 1000W PSU
CPU:
Quadcore AMD Phenom II X4 955 Black Edition 3.2 Ghz
Memory:
Patriot Memory 'Sector 5' G Series 4GB
Case:
COOLER MASTER HAF 932 Advanced RC-932-KKN5-GP Black Steel ATX Full Tower
HD:
IMATION SATA2 64GB SSD (pretty nice)
Well there's your problem. All of these parts are unnecessary for a dedicated mining rig. You definitely didn't need that CPU, or that much memory, or a case, or the SSD, and you could've gone with a smaller PSU to boot (around 800-850W).
A few weeks ago I put together a rig, NEW, for $950, composed of 4x5830s, that outputs 1240 MHash/s (that's average, not peak). So to charge more than that for a used rig that outputs fewer MHash/s, well ... I don't think you're going to find many potential buyers.