There are many honest ancient coin dealers on eBay but when I was starting out with my collection I avoided anything that was not in a certified ANACS or NGC slab.
I've been purchasing from eBay for nearly 20 years. However I only purchase from reputable dealers that typically have a brick-and-mortar shop. A lot of my early purchases were from Ancient Auction House (the owner retired and is no longer in business), Beast Coins, Vcoins, Ancient Treasures, and Frascatius. I rarely buy from eBay any more as I find higher quality, better variety and cheaper prices thru overseas auctions.
NO! I have been burned by their after bidding has closed snipping policy. Life is too short to put up with that. I refuse to even consider bidding there.
I don't believe that is a thing. Bidding on eBay is simple-- bid your max and walk away...or use a snipe program and stop grumbling.
I'll grumble. At Heritage, you can make a fool of yourself and place more bids until the auction is truly over. That's the way it is on all the other auction sites where I participate. I HATE the way eBay runs its auctions. You will never change my opinion.
I have been using eBay as a targeted dealer-based, rather than coin-based buy system, but have been moving more to biddr. I have some reputable dealers (like 3-6) I have pulled from long-time members here, and buy a few from them here and there. There is one in Germany which has some good stuff I grab from time to time (since it's local for me). I am still trying to be a sponge, so am trying to get as many looks as possible at a variety of coins to learn as much as possible, and this helps. Still works better than Reddit, where all of the posts are, "what is this/I just bought this, ID it for me", "is this real", "what is this worth", "here is a pile of stuff which I acquired, what do I do with it" posts.