Our software does not have purchasing ability for classifieds listings. We think Ebay defined the classified ad best on their site here:

http://pages.ebay.com/help/buy/formats.html

The classified ad format is mainly to bring buyers and sellers together and the transaction is completed outside the software installation/site. There is functionality within the software that will duplicate this functionality and still provide more protection to the buyer and seller than the open classified format. We suggest using the auction format but use a buy now only auction. The auction listing will have the buy now (or buy me or buy this or get this…or however you want to label it) button and no make bid button. The first person willing to purchase at that one price will click the button and close the auction.

1. The buyer and seller are protected by the feedback system provided by the site. The classified format affords neither the seller or the buyer to rate the other after the purchase is over. Thus each classified purchase is a shot in the dark unless you personally have dealt with each other in the past.
2. You as the site can charge a final fee on the purchased item if it is a buy now only auction. You still have the option to charge up front by duration or flat rate as you can through the classified format.
3. There is no competitive bidding for the product sold. The first one willing to pay the price clicks the button to purchase it.

You have complete control of the text used within the system. This allows you to label the auctions buy now only feature as anything you wish. Just change the text where ever it appears to what you are naming your feature. Unless you want to allow classifieds also you only need any of the auction editions of the software to accomplish. After labeling changes if you want to make them there isn't much else to do.