Take a gallon jug of diesel with you. Disconnect the inlet and return lines in the engine compartment and hook them into the gallon jug. See how it runs. Make sure you have the $500 deal in place before doing this, as it might change the seller's mind if it runs fine on an alternate fuel supply.
One tank of WVO is unlikely to do permanent damage. 100 tanks on the other hand. . .

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13