sell or trade?

I hate to do private sales, so my last two coaches have been trade in the old one. My negotiations are always just "how large a check do I need to write to get the new coach". I know how much I'm willing to pay so the dealer can structure the deal any way he wants as far as price and trade value as long as the difference is acceptable. In California sales tax is calculated on the purchase price and not the difference between sales price and trade, so a lower sales price with a lower trade is a selling point.