Omega Constellation 'monocoque' Piepan models appear to be undervalued at the moment. Monocoque is a term meaning 'single shell' and the Constellation monocoques can only be opened with a special tool through the crystal as there is no snap-on or screw-down case back.
This is probably one reason why many buyers tend to avoid them. Sellers are reluctant to have them opened, because it means another trip down to the watchmakers at some expense to open the case and photograph the movement. The gold top example shown in this post was obtained for US 365.00 in an on-line auction - an absolute bargain for a calibre 564 quick-set date movement in near mint condition. The model shown retails in brick and mortar vintage watch outlets for between US 750.00 - US 1200.00 depending on its history and condition.
Calibre 564 is regarded as one of the best automatic date movements ever made by Omega, and it's well worth taking a risk on a monocoque if buying from a reputable seller with excellent feedback. Remember it's the quality of a movement, as well as the aesthetics and metal content of the case, that determines the value of a watch. Other goldtop Pie Pans in reasonable condition will sell in on-line auctions for at least US 650.00 - 750.00 even on a quiet day!
If the vendor doesn't wish to have the case opened for a photograph of the movement, advise them that the movement should be a 564 for them to claim authenticity and ask if they will guarantee that the correct movement is in the watch. The above watch was purchased with a guarantee from the vendor that a full refund would be made if it turned out not to be a calibre 564. A quick check of the movement revealed a pristine calibre 564 only in need an oil.
(C) Desmond Guilfoyle 2006