Hello, I'm interested in your tent, can you tell me more about it?
Hi!  It's a great tent.  Perfect for camping as it's waterproof and heavy duty.  I'd like $20
Really? Your listing says $15
Sorry.. I have two. You are correct.  This one is $15.  Would you like to see it?
Sure, but I'd prefer not to pay more than $12 for a used tent. 
How about if we meet in the middle?  $13.50?  It also has a separate area to change clothes or dry off after swimming.
Sounds fair enough. I can come pick it up tomorrow. 
Sounds great.  You won't be disappointed.