There is a loophole in a rule. In the wiki it states that a spawn shop must have at least 8 signs. Well what if u make 8 trade signs saying the same thing!?
It is true that a certain number of trade signs are required to be in a shop, because it is supposed to be a shop. You have to have at least 8-10 signs, but what you buy or sell, or how much of it beyond that, is entirely up to you.
I guess that paying 5k a week to troll is worth it to some players. It is a shame for those that work hard to have nice shops to have to be in the same market and it is unfortunate for new players that walk in the market to see these shops.