As krishnandu said, TDF is simply a convenient medium for sellers and buyers to know about each other. It's not like eBay where the site manages and controls the buying process and also provides insurance against fraud.
There's always some risk in personal deals between people who have never met each other. But judging from remarks made in the forum, most deals are completed satisfactorily (although there seems to be an occasional bad deal). Exchange some communications with the other party and try to judge if he's reliable. Also see if he has received a feedback in the Trader Feedback Thread.
As for payment, there's no rigid rule. Some sellers ask for pre-payment. Some are satisfied with receiving payment after the buyer has received the item.
As an example, when I bought something from avichandana20000 a couple of weeks ago, we worked out the details by email. After that, he went to DTDC but they wanted Rs.605 as shipping charge. He called me from the DTDC office and I said that it was way too much and that they were probably trying to cheat him.
I asked him to try Speed Post. SP quoted Rs.167 and I said that that was OK. He then booked the packages and sent me a scanned copy of the Speed Post receipt. Then I transferred the price of the items plus the shipping cost into his account by online transfer.
You can view details about that sale -
The thread:
Cooler Master 500w Extreme
Packaging:
Packaging
Feedback:
Feedback thread (at the bottom of this page0
Of course, this is just one way of doing a transaction. It all depends on what the seller and buyer agree to between themselves.