Elee, I work in property in Kalkan. In answer to your question, yes 3% from the buyer and 3% from the seller are standard for all estate agents for re-sale properties.
The sales tax is most commonly shared between buyer and seller, but it's often a point of negotiation. There is no rule that says this has to be halved, and often either the buyer or seller will pay the whole 4%. Hope this helps!