It is easy to be judgmental with this type of auction. Most of the stuff sold is true junk. I see several decent cello and rack packs (1984 Topps cello pack with Mattingly on top, 89 Donruss rack with Griffey, etc) in their picture. These are not a bank breaker packs, but I'd be willing to bet they hardly ever, if never, ship out with any of those lots they are selling!
Take a few 20 year old packs (1992 or older) for instance and add in at least 1 that is 25 years old (1987 or older). Figure 15 cards per pack back then on average and that will get you roughly 7 packs, or much less if you get a rack pack or two. That could be a variety of "great" stuff like 1992 Donruss, 1992 Fleer rack, 1991 Topps, 1988 Score and 1990 Donruss and maybe a 1986 Topps pack for your 25+ pack. Add in 2 HOF cards and maybe you have a 1989 Donruss Paul Molitor and a 1988 Topps Bruce Sutter. Is that worth $6.50. I would say probably not! Still, you are getting what is advertised, so the seller has done nothing wrong. I didn't read the feedback, but I have on other dutch lot sellers and they are often filled with comments about how the buyers got what they were promised, but nothing great. That is going to be 99% of the buyers and maybe they slip in 1 decent lot just to show it is possible to beat the odds. Maybe that person is even a plant, who knows!?
If someone would actually take a solid collection, with a mix of really good, decent and even some junk wax and a decent selection of stars, HOFers, vintage, rookies, small sets, graded cards, autographs, GU and such so that you would truly have a random shot at some great cards, then that would be a fun gamble to take, but what seller in their right mind would include something like a 1966 Topps Mays or an unopened 1975 Topps cello pack in a grab pack for $6.50 with no guarantee that they'd sell more than a couple dozen packs because most buyers automatically assume the best you'll get is a 1992 Score pack and a 1987 Topps Tom Seaver card?
Why, because that is what the scammers have already tried to sell us. If you had a collection conservatively valued at $100,000 to liquidate and you could somewhat evenly distribute it across 2000 mystery packs, you would need to sell all of those packs at $5 each to get your money. If you could sell all 2000 packs, you'd get $100K for your collection and the buyers would have a legit shot at getting some good stuff. That would only work if all 2000 lots sold though, otherwise the buyer risks giving away the best stuff in the 100 or so packs they could sell and being stuck with the rest of the junk that won't sell. The same theory on a much smaller scale applies to card boxes in a card shop. Most dealers don't want people to know the "good card" has already been pulled from a box, otherwise nobody will want to buy the rest of the packs.