Good afternoon. Are you interested in the bike for sale?
Hello, I'm interested in your bike, but I'm not sure I want to pay so much for it. How does $20 sound? 
I'm sorry, I can't go that low. I am willing to take 20% off the price, though. That would bring it down to $32. It's in good condition, with stand and training wheels.
My girl already knows how to ride, so how about $25 and you keep the training wheels? 
I'll approve your offer if you can pick it up today.
I can do that. Thanks for negotiating.