There's a good chance the files are still on the computer. Did you check the recycle bin? Even if they were deleted, the files are still there unless there was some serious overwriting of the disk space they occupied.
Take the computer back to Best Buy and tell the manager that if he doesn't pay a data recovery specialist to recover the files, you will exercise every legal remedy available to you.
I think the very first thing you should do is have someone take a full disk image of your hard drive. A backup at this point won't help.
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