As someone into vintage comics and cards myself, I have to admit I'm largely clueless as to the extent of these restoration activities. That being said, I haven't really chased high-end stuff where the likelihood of paying full market price for an "enhanced" item would be high. The most I've ever paid for a single comic is probably only in the $15 to $20 range, and that was probably in the early 90's.
I'm also a lifelong, off-and-on coin collector, and one of the maxims of that hobby is "before you buy the coin, buy the book", meaning become as knowledgeable and informed as you can possibly be before splurging for a big-ticket item, or even not-so-expensive coins. There's a whole variety of unscrupulous activities going on ranging from using chemical or mechanical restorative tactics (a big no-no in the hobby) to outright fakes and counterfeits of rare and valuable coins. As far as cards are concerned, I'm sure Ernie is far more armed with the info and knowledge to avoid being scammed than most of us. Basically if you have enough passion for a given hobby to invest the time to gain the necessary knowledge, you become the shady dealer's worst nightmare.