I have three things to say to that : right on right on right on In my earliest days of collecting, I bought raw coins and low-tier TPG coins on E-bay. I thought I was getting a "bargain". Now that I know a little more, those were consistently mistakes. Never again. All of which speaks to earlier comments of "You get what you pay for" and "there is no free lunch"... right on right on right on
If you are plugging holes, then eBay using trusted sellers or sellers that have been recommended would be a good option. If however you are building a serious collection on a series, Tel-A-Trade and Heritage are your best bets for auction houses.
I used to use Ebay to purchase silver bullion coins, mostly SAEs and maple leafs, before silver went up a lot in price. It was actually possible to buy them at times for an all-in price [bid plus shipping] at melt value or a few cents over. But since silver went up past $8, $9, $10 etc... people are more aware of them and bid them up to significant premiums. Too many people bid crazy-high prices on Ebay for things readily available from on-line and store front coin dealers. Auction fever I guess. The only thing I've purchased from Ebay in the past year are tokens.
I've bought a few coins through Teletrade. My only complaint about them is the pictures of the coins. I've bought a few that had many more problems then were evident on the pictures. Heritage just does a better job. I've had a few coins I bid on that were well above going auction prices but the buyers wanted more and pulled them. I personally don't like the coins I bid on just dissapearing from my tracking list. As far as Ebay. Sometimes you can get a few coins at decent prices , but more then often a good majority of the bidders have no clue of a coins worth and bid outrageous prices. Ebay can be great for selling coins. All of this is just my opinion.