Hello, I'm selling a king bed with boxspring, matress, and headboard
Is there any kinds of stains or anything on it?
absolutely none
As much as I really need this 900 seems too high for me. 
I think 900 sounds fair. I'm including a waterproof hypoallergenic protective mattress cover thats new.
I can really only afford to pay 550 for this at this time
550 is much much to low. This is less than a year old. I'm including everything. A each would be worth several hundred dollars seperately. I can go 875
I just can't justify that when I could get a brand new one for that much. 
This was more than a thousand originally. how about 800. Lowest I can go. The headboard was custon ordered.
Could you throw in a blanket and a pillow?
I could do that.
Then I will make that deal with you for 800