Just contact the seller and let them know that you are new, they should be more than willing to help you out and walk you through your first transaction.
Much depends on the size of the sale and the perceived risk of a loss if things go wrong.
It is always good to make sure a person has good rep on his trader status.
Most small sales can be made via paypal if a person has been on the forum for some time and has positive rep for his trader rating.
For larger purchases one might consider using an escrow service such as EscrowDNS.com or Escrow.com to add a level of security. If the purchase is large one should do additional research like checking on the whois to make sure the domain is registered to the party claiming to own it etc. When in doubt ask questions before hand.
As far as the transfer, the seller should be able to fill you in on the steps as it varies from register to register.
You could post sold BUT you may obligate yourself to the BIN or buy it now price posted. Try first to make an offer vi athe way the seller desires.
Use services such as escrow and look at trader feedback. Learning how to transfer is easy and yes the seller should help. Whole thing can take as little as 1-2 days start to finish depending upon the payment method and registers used.