Once a kit has been opened -- and worse, if work has been started on it -- it becomes a problem for the potential buyer. The first hurdle is missing pieces. Secondly, it's hard to tell about the quality of the work that's been done.
I recently bought such a kit of the Revell Cutty Sark on ebay, for parts. The seller was up front with the fact that the hull was missing. Fine. But many of the rest of the parts looked like they had been glued together by a 5-year-old with palsy. I could hardly believe it! (But at least I didn't pay much.)
Most modelers, I think, would agree that the main pleasure in models is in building them. Completed models only have value if they reflect superb craftmanship -- on a level well above that of the prospective buyer. Otherwise, why bother correcting someone else's mistakes?
As soon as a model is completed, it starts accumulating large and small damage. Such models have to be periodically cleaned and maintained to continue looking good. Storage and display can be a problem -- and shipment of such a fragile item can be difficult and expensive.