As others have said trades are usually only for new. I have traded two vessels but in both cases my used boats were in spotless condition and the broker already had a buyer in mind. Both of those trades were under contract within 48 hours. One was a sail boat, Catalina 36, and one a Downeast style power boat. Depending upon how you negotiate (read very important aspect) trading can work, worked out twice for us.
For our situation it actually put us ahead slightly, financially, because we was now only paying tax on the difference between new and old not on the entire 100k+ sale price and I was able to negotiate down the 10% value deduction for "commission" to 3% & 3.5%.
If your boat is not spotless, and mechanically perfect or near perfect, it won't likely work out well financially to trade. Occasionally a broker will own a used vessel that they took on trade towards a new vessel. Normally they don't like to trade on these but if your boat seems more marketable than the one they own, cleaner, better equipped and more desirable model, they may consider it. Dealers want boats they can move & SELL.
Of course the boat market is almost always good for boats in the top 2% of condition. I secured a buyer for Tim R's Ericson with ONE POST on another forum. Because Tim's boat was SPOTLESS, and in the top 2% condition wise, there were PLENTY of buyers waiting with checkbooks. Tim paid NO commission and had two qualified buyers in the queue behind the guy who bought it. My friend Steve is still very regretful & bemoaning the fact that he missed it. I saw him last week and he has looked at another ten boats since, all junk, compared to Tim's. Steve is a ready buyer but can't find a boat. I know another guy on another forum, Ed, who is also ready to buy but can't find a clean enough, well maintained boat either. Ed has been looking for well over a year.
Tim R. found his new boat, another top 2% boat that was NOT for sale at the time, and worked a deal and now owns it. Very often clean well cared for boats do not even need a broker or hit the market. My friend Bill sold his Sabre 362 this way. The buyer knew of his fastidious nature, called Bill and made an offer Bill could not refuse. If yours is at least in the top 90th percentile, in terms of condition, I would try to sell it.