Here is the system the National Aircraft Appraisers Association uses:
INTERIOR
#10 Rating: Interior condition is flawless. All material, fabric, plastic, carpet, headliner, wood cabinetry, etc. are spotless, with no matting, scratches or signs of wear. Seams are straight, tight and in general the interior looks, feels and smells new.
#9 Rating: Interior is almost flawless and it would meet the #10 rating criteria except for minor exceptions. Carpet at the entry area and in the cockpit and perhaps the pilots, and/or the copilots seats may show slight signs of matting.
#8 Rating: Interior is very clean with no tears, loose stitching, stains, fading or excessive wear on fabric, carpets, plastic, wood cabinetry, or headliner.
#7 Rating: Interior is clean with no tears, major stains or fading or excessive wear on fabric, plastic, wood cabinetry, or headliner. Carpet at entry and cockpit areas may show signs of wear but are not ragged. Stitching is tight, although the seams may not be straight. The interior may need to be cleaned but once cleaned would show well.
#6 Rating: Although the interior has stains, which may not clean up, in general the fabric is in good serviceable condition. The carpets would show wear at entry and cockpit areas. There may be matting of materials on seats with wear noticeable on arm rest and lower seat cushions. There may be stains on headliner and/or signs of material fading. However, the fabric is generally bright with no tears although there may be areas which have had upholstery repairs. A good cleaning may be in order and after cleaning, the interior would look satisfactory.
#5 Rating: Entry areas, cockpit and other high use areas show significant signs of wear and/or stains. Seat cushions, headliner and side panels may have stains, loose stitching, fading, and in general have a well used appearance. Any needed repairs are minor in nature, and the interior may need a good cleaning, but after cleaning the interior would still have a well used appearance.
#4 Rating: Generally the interior has the same characteristics of a #5 rating except for definite need of repairs. The fabric areas exposed to sunlight are well faded and beginning to dry rot. The only way to improve the appearance of the interior would be to install a new one. The existing interior is still serviceable.
#3 Rating: The interior has all the conditions of a #4 rating except that the extent of repairs is excessive. The interior as is, is in poor condition and is not serviceable.
#2 Rating: The interior is not serviceable and the extent of repairs to make it serviceable are not cost effective. The interior needs to be replaced.
#1 Rating: Generally all of the characteristics of #2 with the exception of required repairs to interior structures such as seat frames, chair rails, cabinetry etc.