If you want more for your AW just put an add in Craigslist or one of those for sale apps. Apple is just buying it for the components so if you trade it to them think of it as a $100 trade in discount and not selling your old watch to them.
In terms of upgrading your Apple Watch from the S4 to S5, and your idea of selling it on craigslist, you will not accrue more. Craigslist is the most well-known site for people to haggle hard. If you want to make the most, you probably would have a wider audience like sites like eBay with A demographic of somebody who’s willing to pay your price based off the condition.
Local buyers are looking for the ‘best price possible’, Craigslist is the pinnacle of never generating what you want for the Apple Watch, especially seeing how quickly they depreciate. Swappa and eBay will maximize the profit. [Pending condition, accessories included, ect.]