Here's a picture of the headboard.

Looks like grease marks. Have you ever pulled something from your closet you haven't worn for a while and it had a yellow stain. It is grease that looked like it washed out, but a little was left and turns yellow over time.

Do you sit up in bed to read? Looks like it could be hair products leaching onto the fabric. Especially since the worst places look low down where your heads would hit the headboard when sleeping. If none of those, I'd contact the manufacturer or seller and complain.

@Olychick No, I don't sit up and lean on it often. You also can't tell from the picture, but the stains are all over the headboard, all the way to the edges. If only I noticed this 2 days ago and not missed the return window.

Does the sun ever it? I wonder if that could be causing it. Even if it's out of return window, I'd still insist they take it back. Make a fuss about it; you can even try filing a claim with your credit card company for a defective purchase. Sometimes that works, but have copies of emails you've sent trying to resolve it. Keep records of any calls, too.

@Olychick It's actually on the opposite wall of the windows and I usually keep my blinds closed. It really is baffling what could have caused this. You're absolutely right though. I have nothing to lose by contacting customer service, so I might as well try.

Do you have any pets in the house?

@MiMi Nope