Hello there and welcome.
I think it's great that you are trying to make an informed decision before purchasing. A lot of people make the mistake of not doing that, and either end up supporting a 'bad' breeder, or not even getting a purebred Bengal at all. =/
Marbling is not exclusive to Bengals, it is actually a pretty common tabby pattern. Though the more 'quality' marbled Bengals should have a very distinct horizontal flow to their pattern, not a circular or bulls-eye one. Spotting is also not exclusive to Bengals, there are a few other breeds that can be spotted, including your generic domestic tabby. Rosettes, however, are the one pattern that IS unique to Bengals (though even a Bengal mix may have rosettes, due to the one Bengal parent).
Based on the pictures of these kittens, it's a bit hard to tell. These kittens do have very ticked coat, which isn't necessarily a bad thing, but their patterns aren't very bold/contrasted either - especially the spotted one, you can hardly even tell they're spots (I've seen regular mixed breed tabbies with better spotting). They also do have awfully long/pointy ears for Bengals. To be honest, if they are full Bengals, I would wonder why someone would even be breeding Bengals if this is the 'quality' they are producing.. =/ That may sound harsh but it just makes me wonder, why are they breeding? Clearly it's not to "better the breed" or to produce quality examples of the breed. So that makes me wonder if it's for the money, which is never the right reason to breed any cat.
As far as the registration papers go, all pedigree kittens should come with these no matter what. Doesn't matter if they are just going to pet homes, the papers are a proof that you have gotten what you paid for. I would seriously question the ethics of any breeder who doesn't register all of their kittens and provide new owners with papers, no matter what. There is never any good reason to not register pedigree kittens, any reason you get would just be an excuse which leads to the breeder not doing things right.
$500 is definitely on the lower end as far as Bengals go. But I guess, that depends on where you are too. Where are you located? Not always, but sometimes an unusually low price can be an indicator that a breeder is cutting corners in order to sell more kittens at a cheaper price, therefore turning a higher profit. It costs A LOT of money to responsibly breed Bengals and carry out all the proper heath testing and everything else. Do you even know if this breeder tests their breeding cats for HCM/Pk Def? Both are common problems in the breed (you should research into these if you haven't already). Do you know if the kittens come fully vaccinated for their age and with health certificates from a vet? Is their a health guarantee? Will you have a spay/neuter contract, or would they already be fixed when you get them? These are all important questions to ask.
I can appreciate that you are hoping to find Bengal kittens within a price range you can afford, but please be careful that you do not end up supporting an irresponsible breeder (or a BYB) in order to save some money. Not saying for sure that this breeder is one of course, just saying to arm yourself with all the information so you can make an informed decision.
Also, have you looked into Bengal Rescue groups at all? We have one here locally that seems to have at least a dozen Bengals needing homes, at any given time. They even periodically get kittens in. Might be something worth looking into if you want a Bengal without spending a lot (plus you'd be saving a life
). Also breeders will occasionally have ex-breeding cats available for a much lower price, sometimes as young as 2 years old.