A real estate market under siege from subprime mortgages and the credit crunch could be hit with another twist. The bankruptcy case in Richmond of LandAmerica 1031 Exchange Services Inc. might harm the market for a key tax shelter used by people who buy investment properties. The consequences could reach far beyond central Virginia and the 450 customers of the now-closed exchange, which is a subsidiary of Henrico County-based LandAmerica Financial Group Inc.