Many times the seller will sign off the aircraft on the bill of sale and leave the buyers name off, and that bill of sale is passed along from buyer to seller a couple times. but the last registered owner fills out the back of their registration with the name of who they sold it to.
When the newest owner tries to use the bill of sale that was left blank by the first buyer, the FAA already knows the aircraft was sold, and who the buyer was. The FAA will not register the aircraft to the newest buyer with out showing a bill of sale for each ownership transfer.
Any body dies in the chain and you can't show a legal bill of sale it will require a long and involved process to show the aircraft ownership transfer was legal.
It really gets involved when the last registered owner is killed in the aircraft, he was divorced, and the wife got the aircraft thru a legal court settlement. ( no bill of sale) and the insurance company took custody of the wreckage (no bill of sale) then informed the FAA that the aircraft was destroyed, then sold it to a salvage company ( no bill of sale) who sold it to you as a restoration project. (no bill of sale) because after the FAA de-registered it it is no longer an aircraft.
Now do you know why you need the CD? it will show it was de-registered.