In general I agree with the above, but it really depends on the dealer, and your relationship.
In August 2015 I bought a new end of model year 22' trailer with double slides from the local dealer, who I had previously bought a couple of used trailers from over the years. I was never really sold on slides (see all the threads about the problems with them), but he didn't have any units without slides (new or late model used) and gave me a really good deal, better than anything I could find on the internet. Last year he got a 2014 21' no slide trailer in trade. My trailer booked for more, and I assumed at best he would give me an even trade. He actually wrote me a check for $2000 for the difference in value. Plus, because I didn't "pay" for the 2014 trailer I saved almost a $1000 in NY State sales tax over selling the 2015 and paying cash for the 2014.
Earlier in the spring I had tried to sell the 2015 on LetGo and a few of the local Facebook "garage sale" pages. If I had sold it for what I was asking, with no negotiating, and that doesn't happen, I would have come out $1000 ahead. I got a few inquiries, but nothing serious, even though I had priced it between trade in value and retail value. The dealer sold it within a week and I subsequently got to meet the new owners at the State Forest where I boondock when they came for a weekend. They had downsized from a fifth wheel since it was just them, and occasionally their grand-daughter.