I keep seeing posts advertising they are looking for or selling CSC for SP, or trading CSC for items--but they are often amounts that you can't get in the store, and I can't for the life of me figure out how you accomplish this.

I've checked the help sticky on CSC and the Cash Shop, I've browsed the advertising forums, and I've looked in the Cash Shop itself, since I've seen card items in there--but those are in static increments.

Am I just missing the obvious? (It feels like I am.)

You can offer csc on trades! If someone is offering sP for csc, both people set up a junk trade an offer on each other's lots (Like person A wants the csc, so person B offers it on person A's lot, and person A offers sP on person B's lot). If they're trading for items, they can just offer csc on the trade with the item.

OOPS SORRY TOO LATE

Ah! See, I knew it was something I was overlooking. Thanks so much, both of you. :)