T/T stands for "telegraphic transfers "which means paying cash via draft.
These days because of some imposed sanctions against Iranian banks and problems of L.C, TT has been the commonest way of foreign exchange transfers.
In this procedure the foreign seller requests 10%-50% of the whole (according to what is in proforma invoice) as a prepayment. 
Pre- payment is totally negotiable so no particular rule exists for the amount. The price won't go higher than 50%, also 30% of can be a reasonable price in this case.
Buyer must pay the agreed price to the seller's account.
Due to sanctions against Iranian banks, you can go for this procedure via exchanges. 
By receiving prepayment via seller, he must start preparation and loading goods.
By providing copy of bill of loading papers and other transportation documents (via seller), in this step, he can request the rest of price. 
By paying the balance payment (via buyer) seller must send the original documents of transportation via TNT,BRE and ARAMEX.  
This way of business can be so risky too. So it is suggested to check seller's accuracy by checking them by chamber s of commerce and embassy's commercial sections.
During the past years and sanctions which have been imposed against Iran , foreign exchange transferring has been one of the most stressful points in Iran's  business procedure, but until the complete removal of obstacles you can use the ways below .