All in 1 stereo and nav?

[mak]

Just about to buy an all in one double din head unit with sat nav, looking at the after market options they all seem very expensive (on ebay) but i was just wondering if the double din units from another make of vehicle could be used in any car say for instance a vw origanal unit with sat nav could be slipped into any double din space and wired up? It seems these units are much cheaper to buy on ebay than after market units.