Breyer model horses without boxes?

Few collectors care about the box, and once the horse is out of the box the box is worthless anyway. (The exception is very old boxes from early models, which a handful of collectors are interested in.) You don't need to reduce the price for a lack of box.

However, those prices for flaws may be incorrect. Once a model has a few rubs, or even one very visible one, it's worth drops to "body" price. (The exception is if it is a rare model.) It is mostly only good for customizers, and worth between $15 for a large, really nice mold and $1 for smaller, disliked molds. Chips lower the quality drastically, and even just one may cause the horse to become a "body."

I would research on www.modelhorsesalespages.com and ebay for selling prices for models you have for sale. The second hand market is very slow at the moment and unless they are hard to get models that are popular, or are very cheaply priced, alot of models will not sell. The purchase price is no longer any guide to selling price for most models.

Chips and major rubs (down to plastic or primer) make it a "body" in my opinion - that is only suitable for repainting. This will really lower the price anyone is willing to pay. There are exceptions of course, for some special runs and very rare models, but most regular run models already have many available for sale.

Do your research so you know what your models are worth, then make your decision on asking price.

You can try to get small, glass or plastic, boxes to put the models in. That is really all I can think of. You could get fly spray or keep your window and door closed.