First of all, congratulations! To mark your listing as sold, go to your My Account page, click the My Active Listings link and click the SOLD icon next to the listing you wish to mark as sold. When you do this, three things will happen:
Your listing will have a sold indicator on browsing pages plus a large SOLD sticker image on the ad page
Your contact and user account information, listing contact form, and listing price are removed from the listing page.
Your listing will be removed from the website in 3 months