There's no universal grading method. Thus, terms such as "Excellent" or "Mint" can be highly misleading.
I've sold a lot of vinyl on Ebay and have 100% feedback. I've learned one invaluable secret: Describe your grading scale in every listing (easy to find them on ebay or other sites - I've attached mine below), then CONSERVATIVELY grade your own records accordingly. Show your own scale, don't use Goldmine or some other obscure (and variable) reference. That way, there can be no subjectivity as to what a grade means, as long as you are being honest (and if you are not honest, don't sell vinyl!).
Fact is, 90% of records are not valuable - except to listen to! One important point to remember - If you have a great condition copy of something that was often trashed (Dave Brubeck's Time Out for example - it was a great party album and most copies are splashed with beer stains and have had joints rolled on them, etc), it's likely to be valuable. Conversely, most (not all) classical is not valuable even in mint, since it was typically well-treated by its genteel owner. (The big exceptions to this are certain records from certain classical labels - Mercury Living Presence, RCA Living Stereo - that are in VG+ or better.)
Beyond that, look for Beatles, Hendrix, Zappa, and mono jazz from the late 1950s, and you will have covered most of the stuff you can sell. Having said this, there's nothing wrong with putting it out there - you may find a buyer.
Lastly, don't even try to sell something that is not in at least VG+ condition, conservatively graded, using my scale as a reference.
Note: I usually just describe the cover in detail - I don't use a scale for that.
Doc's Vinyl Grading Scale:
Vinyl graded using the following scale: 1. New (Factory Sealed); 2. NM (No scuffs or scratches, looks and sounds new); 3. VG+ (Visible imperfections that generally don't affect the sound quality, except for a few ticks, often at the beginning of each side); 4. VG (Possible light scratches, plays good but some pops and clicks); 5. VG- (noisier clicks and pops, most noticeable at intervals between songs), 6. G+ (plays through but considerable poppy noise, good for a backup copy); 6. H (hammered - noisy throughout and/or skips).