I'd hate to think the seller tried to slip one past you, but at least they refunded you without a fight. I'm glad you were able to successfully work on it so now you have a functioning lens and a little experience with what's inside.
My only caution is to keep an eye on it and twist the focus ring back and forth before each time you mount it to make sure it still turns easily. If sand made it down to that gear there's a chance there's more in other parts of the lens, and you wouldn't want it to cause issues with your cameras AF motor if it's starts to jam again.
If you notice it sticking again you might want to proactively take it apart again to get any other bits of sand out. Regardless, you should feel proud of what you already accomplished. That's far beyond what most people feel comfortable doing.