Hello. So you're interested in my phone case?
Yes, I see you are looking to get $10 for this case, Would you perhaps go lower if I come pick it up today? 
Sure, I'm open to negotiation. Have an offer in mind?
I was thinking in the ballpark of say $7?
Make it $8 and it's a deal if you can get it by the end of the day.
Make it $7 and I will leave right now and help you pick a better team to root for.