I have a ton of equipment I could sell, but I've kept it in case I make a created player (I don't really play RTTS much.) What's the advantage of having a Created Player? Is it worth the time and effort to level up? Or can I safely sell my equipment for stubs?
I have a couple of answers to your question. In relation to other modes available within the game, an RTTS created player has no real advantage other than possibly giving you an opportunity to practice playing against the computer, which is possible in other modes. The answer to whether a created player is worth it is wholly dependent on you, and what you consider worth your time. Personally, I enjoy creating a player and progressing through the career. However, other than some personal goals and trying to win, there really is not any objective other than to play as well as you can. At the end of the day, give it a try, if you don't get any enjoyment from it then it's probably not worth your time.
If you plan on playing Diamond Dynasty and online then yes definitely. Make a switch hitting SS, level them up and stack them with equipment. You won't find a better SS in the game. If you just plan on playing online I would sell everything and buy stuff for RTTS