Unless you're transferring the license of Parallels, Windows XP and the other software you mentioned over to the new owner, you can't really include it legally nor advertise that the machine comes with it, since you're then giving them pirated software which won't work for them in terms of support, upgrading and could even get them (and eventually you) in legal trouble.
You should do a clean install of the OS and then turn the machine off after it restarts and gets to the part about choosing a language. If you have the original discs that came with it, Install 10.6.8 from them and give them to the buyer. They can opt to keep it at 10.6.8 or upgrade, but the software will be theirs.
You might get more for it if you include Parallels, but you'll need to at a dead minimum include the DVD you installed it from and remove all traces of your own install.
