We are working some details out with the achievement tracker right now and we should have a tutorial up around the weekend
As for the blog, it should be the amount of pictures per post, does that make sense?
Everyone has a different pricing idea really; it can depend on the breed. Popular breeds like Warmbloods sell and are worth more to players than unpopular breeds; people pay more sometimes for rare-colour horses, and then there's things to take into consideration like the conformation, any custom markings, a background personality made up for the horse etc.
Most people prefer sales because everyone has a fair chance; auctions can be tricky because of time zones and newer members obviously have less money than older members. So the reason you want to sell horses would probably be the key to working out how you want to host the sale; if you want to earn money, an auction would probably work better