With the conditions, you're pretty much trusting what the distributor says. You learn that some of them are more accurate with grading than others.
As for saving money, the main way you do that is with the firearms that are still in good supply. For example, stuff like Mosins will be fairly inexpensive, since the dealers won't get them any cheaper than you can, and you won't be paying their markups. On items which are really only found from individual sellers (there aren't large stocks being imported, you just found one offered for sale), you aren't likely to save a massive amount, but you can cut out the whole middleman.