on top of what was said above the seller should edit the original post and type in at the top of the post SOLD and maybe delete the ad text/photos...then add a post at the end of the thread that says sold as well just in case someone missed it the first time...myself i like having the old ads around just to be able to see what things are selling for so i would probably not delete the ad text just add SOLD at the top of the ad and post SOLD at the end...but once things are sold most people don't even bother to do this which is frustrating sometimes
edit: i never clicked "use full editor" before...there you can change the title and description of the thread to "sold ______"
the best way to handle it that i have seen on other forums is to let the creator of the for sale thread "close" the thread when it has been sold (i.e. not delete the thread, just make it so no one else can post in it)...i think this option is only made available in the classifieds section...i usually still add the SOLD at the top and the last post before i close them though