I don't own a shop, but 2 of my best friends do. I know from first hand experience how tough it is to keep tandems on the floor of a shop for sale. Most tandem buyers who buy from a shop are going to order exactly what they want. Year old bikes end up being sold at cost or less. In the interest of helping the shop promote tandeming you should try to work out something that is a good deal for both of you. I don't know how big this shop is but $900 is a big loss for most shops. IMHO a good deal isn't benefiting from someone else's mistakes no matter how careless.