No anexo do manual tem essa informação:
A player who has already built the Spare part storage and decides to build the Trader as well must place the Trader on top of the Spare part storage, thus overbuilding the Spare part storage. (Do not return the Spare part storage to the general supply.) He cannot use the Spare part storage any longer and does not get any points at the end of the game for it. The same applies if a player has already built the Trader and decides to build the Spare part storage. (In general, Furnishing tiles cannot be overbuilt.) (página A4)
Acho que foi a solução para resolver o problema do combo.