If I transferred my 360 account over to the Xbox One and ended up selling it, then repurchased a new one, will I be able to transfer my new stuff on my 360 over to my new Xbox One again?
Per the Rockstar Support page on transferring GTA Online characters and progression, specifically the last two bullet points of the "Please note" sections:
You will still be able to play with your existing characters on Xbox 360 and/or PlayStation®3, however any progress made with versions of the characters on those systems after a transfer will remain on the those systems only.
You can only transfer a set of character and progression data from PlayStation®3 or Xbox 360 one once.
You could resume playing on your 360 after selling the first XBox One (Minus any progress you've made on the One), but you could not transfer from your 360 a second time.