1) Yes, it really is a mixed bag. One thing to do is to look at what other cards the seller has. If they have say 75 T206s and they're all graded except for one, that would probably signal that that card has a problem. Some sellers might just take whatever they get the way they get it and move it along as quickly as they can.
I wouldn't stick to just big sellers who specialize in cards. Most of the deals I've gotten have been from sellers who don't do just cards.
If you're looking at the PSA 7-8 range though it will mostly be bigger sellers, ad the cards are few and far between. The best I've done was an SGC 80 (6).
2) Bad photos are a blessing and a curse. On the one hand, they can be an indication that the seller is hiding something. On the other hand, it can also be an indication of a seller who just can't be bothered to take a decent picture or scan. Just like the grade/ungraded thing, I look at the sellers other items. Great photos except the one card? That's a red flag to me. All the photos blurry? If things don't get out of hand it could mean a pretty nice mailday in my future. (One of my better old bicycles was poorly listed with awful photos. But they were good enough to spot the rare derailleur it has
)
Overall, I take a chance on something from what might at first seem to be an iffy listing, but being cheap I don't often risk much. I must be a good guesser, as most of the stuff I've bought has been just fine, sometimes way better. I can't recall returning anything either, but I'm pretty easy about stuff, especially if I'm taking a chance on a bad picture or listing.