Here is my take on your questions:

1. I normally take a 1-or-more-out-1-in approach for I have about 20 watches and a wife in my collection.  Whether I sell or buy first depends on the urgency of the incoming one (auction? limited release?). The guilty feeling is that I have way too many watches as my ordinary rotation. I love all my watches but some just dont have enough wrist time and that is a shame.  They should roam free on a wrist rather than locked in a box. For instance, I am always in casual wear in recent years and some dress watches do not pair well.
2. Selling (not flipping) is always a good idea when you have more than 10. Your taste changes over time, for sure, except a few timeless models.
3. C24 to me is a good place for this hobby yet the cost is relatively high.  Secured checkouts worked well for me so far.