There are a couple of ways you could do this:
Use animation triggers to make the sold appear when the item is clicked (but
this only lasts for the duration of the slideshow.  So if you exit the show,
everything reverts.
Use VB, see:
http://weblogs.asp.net/stevencohn/archive/2005/01/12/hide-shapes-on-a-powerpoint
-slide.aspx

As Glenna says probably triggers will do this as long as the change is only
until you close (probably?). There's a tutorial here on our site and if you
google there are more out there.http://www.pptalchemy.co.uk/Triggers1.html

If you do want to use vba to make semi permanent changes to the slide be
aware that the vba snippet in the link won't work during a presentation but
only in edit mode. You could use something like in show mode

sub hideme(oshp as Shape)
oshp.visible=false
end sub

to hide the first shape when it is clicked and reveal the "sold" item.
Sometime code using .visible clashes with other animations (a bug?) so its
best to make the slide animation free.