I am looking to buy an OEM Sirius unit. Let me know if you have one for sale.

I would give Elk Grove Nissan a call. Ask for Chris or Sal and identify yourself as a ClubTitan member.
They will give you a 20% discount. I have my XM unit on order and I got all three pieces (receiver, antenna and mounting bracket) for $326.40.
There number is 916-405-5000.