Mint condition.
This is used to describe an instrument that is in the same condition as if it has just left the builder's hands (or the factory, if it is a production instrument). In other words, the instrument is both cosmetically and functionally perfect. There is no such thing as "mint condition, except for a small bump on the headstock" or "mint condition, considering it's age". In order to be mint, an instrument has to look like it was just finished and then placed on a soft velvet pillow! Dust and fingerprints are okay, as long as they can be easily wiped off with a soft cloth.

Near Mint condition.
This is an instrument that looks very close to mint. It has probably been played very little or perhaps even not at all. It can have visible signs of light use, such as very light fingernail scuffs on the surface, or even one or two very light impressions from bumps (but only in the finish) or light scuffs or scratches (again, only in the finish). As far as hardware, some plating may be becoming slightly dull (tarnished) on the edges, and perhaps some extremely light wear on the frets might be visible under very close inspection. But nothing unattractive should be very noticeable if an instrument is refered to as "Near Mint". In addition, all playing surfaces (neck and fingerboard edges) should be free of any bumps or scratches.

Excellent condition.
An instrument with light finish wear from playing. In other words the instrument can have a few bumps and scratches, but only minor ones - the kind that can be reasonably expected to accumulate through everyday playing. Excellent condition instruments show signs of use when examined closely, but from a distance these instruments should still appear pretty clean. There may be some finish checking (crazing), especially in older lacquer finishes. There shouldn't be any distracting bumps or scratches (leaving deep impressions in the wood) anywhere on the instrument, and though the playing surfaces of the instrument (back and sides of the neck, the edges of the fingerboard, frets) can exhibit some light play wear, these areas must clean with no significant (distracting) bumps or scratches that can be felt while playing. Cosmetic imperfections such as minor "buckle rash" or light bumps/indentations on non-essential parts of the instrument (the back, edges of the headstock, or the bottom of the lower bout near the strap button) are noted, but generally aren't considered as important as the playing surfaces.

Very Good condition.
More wear here. The instrument is still perfectly functional as a musical instrument, but visually there are imperfections such as: some of the original finish may be gone, bumps and scratches that may extend down into the wood, some body or neck wear (but nothing that detracts from the playability of the instrument), and so on. Basically a "very good" condition instrument looks like it's been played a lot and it can even have non-original parts (as noted), but it hasn't been abused and there's nothing in the condition of the instrument that keeps it from playing nicely.

Good condition.
This is a term we don't like much - "good" condition instruments are typically starting to look pretty bad (to us, anyway), and they sometimes have some minor problems, obvious alterations, or missing parts too. For this reason we don't generally take in instruments in "good" or lower (fair, poor, or ???) condition. Well, maybe sometimes depending on what it is, but not often.

Other terms:

Brand New.
This describes instruments that has never been sold. In other words, it is straight from the manufacturer and comes with the manufacturer's warranty. There are no grey areas here - either an instrument is "Brand New" or it isn't. However, please note that "Brand New" describes only whether the instrument has been sold or not, and not necessarily it's condition. A instrument that has hung in a store for a while can be "Brand New" but still have some signs of use on it - one shouldn't assume that just because an instrument is "Brand New" that it is also in "mint condition". For this reason, BunnyBass always includes a rating of the actual condition of each instrument when it is "Brand New".

New Demonstration (Demo) Instrument.
This refers to an instrument that has never been sold at retail, but may have been used as a showpiece or demonstration piece by the manufacturer, dealer, or some other representative. The instrument may be in "mint condition" or not, this descriptive term is just used to let the buyer know that the instrument hasn't spent it's life laying in it's case. BunnyBass always includes a rating of the actual condition of each instrument when it is "New Demo".