How are used models' conditions classified?

MB (Mint and Boxed)
Model in absolutely top condition, in original pakage. (Practically New). These are usually sold at their 100% price, often set by collectors and the known market value of that particular model.

M (Mint)
Model in perfect condition but not necessarily boxed. 60-80% value.

NM (Near Mint)
Model with minor traces of age or usage. 50-60% value.

VSC (Very slightly Chipped)
Model with minor defects in paint and or parts. About 50% value.

SC (Slightly Chipped)
Model with clearly visible defects in paint and or parts, but still in good overall condition and all parts present. 30-50% value.

C (Chipped)
Model with major defects in the paint and or parts, but with its major parts still present. these are sold at 20-30% value.

AT (Almost Trash)
Model with major defects in the paint and one or more major parts missing, such as doors, wheels etc. 0-20% value.

All of the above can gain as much as 25% value if boxed It is also important to note that these values are not in any way a given rule. It all depends on ofcourse how much a collector really wants a model and is willing to pay for it and how much the other is willing to accept before parting with it. This also applies to RP models (repainted models).