What proof does the seller have that the engine only has 80k miles on it? Probably none. Pull the valve covers and see if its all gummed up in there from not changing oil. If he can get it on a stand pull the oil pan off and look up into the bores. Look for scoring and scrapes. At 80k miles those bores should still look great. Maybe pull a main cap and look at bearings. Are they all scored up or nice and smooth? Do not flip the engine upside down before pulling pan; if you do anything laying in that pan will now be up in the cylinders or cam shaft area. Keep the engine upright.
$300 is a good price for that engine even if it needs a little freshening up.
