Another user had a similar question - so do check out the answers there.

https://apple.stackexchange.com/questions/12996/how-to-install-apps-to-snow-leopard-after-a-fresh-reinstall-and-still-keep-the-ma/13012#13012

In a nutshell - you can make a "test" account to delete the personal information and apps you wish to not transfer. You can then run all updates and leave things just as you wish and restore a first boot experience to the new buyer.

Do remember to deauthorize iTunes on that machine, unpair bluetooth and networks / forget network passwords. If you wish to be exceptionally thorough, you might delete old log files and crash reports. The console app will let you poke around and see if any cause you concern. I wouldn't just delete the entire directory (as some of these directories have special permissions and the system will fail in hard to realize and fix ways if the directory containing a log file is missing.) Typically the log file itself will get recreated at next boot, but the programs that maintain the files generally assume the directories have been set up properly and they won't make entire directories. Just realize you are mucking around in the bowels of the unix system my modifying or deleting these files and directories may make the mac you sell less stable or cause issues to the new owner who may be ill equipped to solve.