market price

So, i've been wanting to start a trade shop recently and I'm checking in the trades forum about market price stuff. My question is: how do I know how much an item is worth? For example, a summon item. Where can I look for to see the "general" pricing of a certain item? In another case, shinies and albinos too. I'm really confused about the pricings of pokemon. Is there someway to set a price in an acceptable range? I don't want the prices in my shop being way cheaper or too expensive.

I've seen people following the prices from the market board (ie the cheapest listing and sale history) And as for pokemon the best table/guide I can imagine using for this would be this table But these are just guidelines, you are free to pretty much price anything however you like or feel an item/pokemon is worth. :>