Models can be graded or valued in several ways depending on the purpose. For trading or selling purposes, Ebay is a good example, models are graded on their packaging and condition. Models can be "New in Box" (NIB), "New in Package" (NIP), "New", "Like New", and more. The highest level is called "Mint Condition". Keep in mind that this doesn't mean the model is still in the box however, this means the model has been removed from the box and has been on display/used/etc. Some models are removed from the box with flaws already. Unlike many toys, Breyer models normally do not have more value in the box. There are some issues with the per-2000's package that causes harm to models within boxes after a period of time by rubbing off the finish or paint against the box, therefore lowering the model's value.
Common flaws include rubs, scratches, breaks in ears, tails, or legs, yellowing, seam splits, and bent legs. These flaws can come from use or storage. Flaws can also come from the factory. These flaws include, but aren't limited to, off-target painting, sloppy detailing, bent legs from improper cooling, or badly sanded seams. Though factory flaws aren't all bad. Some collectors search for ones with painting flaws on purpose to collect them as oddities.
Knowning the rarity of a model also helps when grading it. A model is considered "rare" if only sold for a limited time in the past. This means there are very few in circulation. Models released in very limited numbers are also considered "rare". In extreme cases, there are models that are one of a kind (OOAK). They are given out as prizes or auctioned at Breyerfest. These are the most collectible models.
Model horse shows are a good way for collectors to show off rarer models. In a show, models are competing for one of two grades. One grade is called Live Show Quality (LSQ). The other is called Photo Show Quality (PSQ). LSQ means that the model and tack on the model depict a real animal and is in good condition. The model will be inspected from all sides for any flaws (including those from the factory). PSQ is the lower of the two grades because the horse and tack are only judged from one side in a photo. The standard is also not as high for PSQ.