My records are graded using the Record Collector grading system.  I have been listening to and grading records using this system since the mid 1970ss.  I generally only sell records that are VG or better unless a record is particularly collectable.

Gradings are as follows: 

Mint:

The record itself is in brand new condition with no surface marks or deterioration in sound quality. The cover and any extra items such as the lyric sheet. booklet or poster are in perfect condition. Records marked as Sealed or Unplayed should be Mint. 

Excellent (Ex):

The record shows some signs of having been played, but there is very little lessening in sound quality. The cover and packaging might have slight wear and/or creasing. 

Very Good (VG):

The record has obviously been played many times, but displays no major deterioration in sound quality, despite noticeable surface marks and the occasional light scratch. Normal wear and tear on the cover or extra items, without any major defects is acceptable 

Good (G):

The record has been played so much that the sound quality has noticeably deteriorated, perhaps with some distortion and mild scratches. The cover and contents suffer from folding, scuffing of edges, spine splits, discolouration, etc