I saved myself $850 standing in the street in Ojai just about to shake on a deal for my 71 K20 burb when I noticed the 5 year old tags..
fortunately I had this in my phone and the seller realized his mistake and took the back reg straight off the top.. stand up guy!
Vehicle Registration Fee Calculator
don't know if that'll work on a lien sale/auction but worth a shot.. you'll need vin, plate, and zip but uncle should have all that..
good luck!