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Buying a mortgaged house
Just as a renter doesn't care what the landlord's mortgage is, the buyer of a house shouldn't care what the seller paid, what the current mortgage is, or any other details of the seller's finances.  Two identical houses may be worth $400K. One still has a $450K loan, the other is mortgage free. You would qualify for the same value mortgage on both houses. All you and your bank should care about is that the present mortgage is paid or forgiven by the current mortgage holder so your bank can have first lien, and you get a clean title.  To answer the question clearly, yes, it's common for a house with a mortgage to be sold, mortgage paid off, and new mortgage put in place. The profit or loss of the homeowner is not your concern.