Mint Condition  new condition -- fully functional -- no scratches, dents or rust -- all original parts -- 100% original finish -- in a word... Perfect.

Excellent Condition  appears new -- fully functional -- all original parts -- minor scratches -- no dents or rust -- 90% original finish or more.

Very Good Condition  minor play wear -- fully functional -- some scratches -- no dents -- may have surface rust, but no pitting -- finish must be 80% original or more -- original parts, with the possible exception of bolts and screws, (the replacements must be similar to the original bolts and screws).

Good Condition  fully functional -- may have replaced parts -- surface rust and minor pitting -- some scratches -- minor dents, but gun displays well -- finish must be 50% original or more -- acceptable to most collectors. 

Fair Condition  scratches and dents -- rust and pitting, but no holes -- minor parts missing -- finish is less than 50% original -- gun may not operate, but appears as though it would -- does not display well.

Poor Condition  rust, pitting and holes -- missing parts -- does not operate -- does not display well -- can only be used for spare parts.

Note:  All conditions are subjective.  For example, a gun may have less than 80% bluing, but still be labeled "Very Good" if it is rare and I consider it to be in "Very Good" condition relative to other guns of the same model that I have seen in my years as an airgun collector.