If you're looking to collect, not sell, then Ebay is just fine. Plenty of buyer protections in place. If you by a book that arrives not matching the pictures/description, you're protected. I will say descriptions can get a little sketchy, but if you thoroughly read what you're buying you have nothing to worry about.
Regarding slabbed vs. raw for collecting, that's really all on you and comes down to your preference. Do you want to read and/or examine your books? Go raw. If you want to know for sure a book is a certain grade and admire it in all of its slabbed glory, go for slabbed.
If your goal is to flip books for profit, that's a way more in depth and complex discussion.