As others have said, you will need to install the spinlocks unless your car originally came with a hard top or the speedster humps.
$1000.00 is a pretty good price for a Boxster top. If it's local enough to pick up that's a big plus as shipping them is a pain and $$$. I paid $1,100.00 CDN a few years ago for mine but it was already painted to match my car. I've never seen them as low as $750. The 996 tops are less costly on the second hand market, but of course do not fit Boxsters.
Here is a link for a DIY spinlock install that will walk you through the procedure as well as show what to look for to check if your car is already equipped with them.
http://boxster.wikia.com/wiki/Hard_Tops
And a link for spinlocks ($74) at Suncoast Porsche.
http://www.****************************************************.com/product/986HSL.html?Category_Code=POTEBXEX
Suncoast also has the interior black plastic hardtop latch covers that were missing from mine.
The spinlocks are a bit of a pain to install and get the top adjusted properly. It may take a bit of fine tuning to silence all or most of the squeaks and rattles. The seller would need to be a pretty understanding guy to let you remove his spinlocks as it requires the temporary removal of some of his car's interior carpeted trim panels. But it can't hurt to ask.
I was able to slowly drive 10-15 miles home w/o spinlocks after picking up my top but I wouldn't recommend driving at highway speeds.