Trade Market

Can someone tell me why an item when the offer is that it costs 4 exalted orbs and you send them is not approved.

what kind of item? it may be pricefix

Trade is more like a bazaar, you write what you would like to get and then see what others bet.
There are some who writes prices on items but never want to sell, just for ppl looking, so they are trying to manipulate the market.
And there are other which are willing to trade even if you just offer half of their price...

So by design its almost a bit of a gamble involved.