Let me be the first to say welcome to this forum.
The members on this forum have been here for a long time and there have been many of us who have sold rims and tires on here. The average price for OEM 18" rims and tires together is around $500-600. You are new here so I hope other members don't give you too much sh*t for asking a ridiculous price but for the future it would be best to view past threads to see how much these tires are selling for. A more reasonable offer would be for you to include the rims if you want the $900 and that is only because they are the new style rims. If you had the older style, forget about it lol.
Remember first impressions are everything.
GLWS (Good Luck With Sale)