I'd have a crack at it... :-)
In short, you will make VERY heavy use of PHP's mysql_connect, mysql_selectdb, mysql_query, mysql_fetach_assoc and mysql_close functions. If you design your DB schema properly, your queries will be simple, and fast. Heavy use of the PHP include function is also paramount to quick deployment of such a project, as it enables you to modularise your chunks of code, eg. one chunk for querying and displaying users, another for adverts, etc. You'd just include then when appropriate to do so.
Whilst it is certainly true that dynamic sites are far more complex than static ones, it's all in the planning... spend a day with a pad of paper and map out your DB schema and your site flow - then you can identify where your data will need to flow in and out. You'll end up with a fairly obvious action plan that can be dealt with one step at a time in quick succession.
As for payment processing, BadBob is right: you'll need a third party payment processing solution, or just take money through paypal (which is a kind of half way solution)
If you need more help, just write back :-) Good luck