I would personally have no problem buying a used U-Lock. If you just buy the lock, and the alleged thief hasn't seen your bike or know the locations you frequent and would lock up your bike, then they have nearly no chance of ever stealing your bike.
If the lock itself isn't somehow modified, i.e. painted, to make it easily identifiable, then I don't see a problem with it. Also, you live in Portland, there have got to be dozens or hundreds evo minis around, if you were in some tiny town, with only a few bikes you'd be at a higher risk, but still....
And like JB said, this would be a pretty wacky bike theft scheme. Maybe it is a thief who has gotten bored with using bolt cutters or swiping poorly locked bikes, and now they are looking for a real thrill. Riding all over the city, trying their key on all evo minis 'til they hit pay dirt!