There is clearly a demand for sharing craft ideas and designs. Look at KerbalX! In a multiplayer online version of KSP, I could envision well-funded, but lower tech players saving enough funds to buy vehicles with improved tech that are sold by other players. This would be similar to COTS efforts by NASA. This also allows players to buy lifters to get payloads into orbit. The ability to do so is a nice middle ground between contracting other players to lift your payloads and launching all payloads yourself with lifters that are available to you.
Craft purchased from other players could not be modified in the VAB/SPH except for action groups/staging.