There are some grey area loopholes, like getting keys from bundles and selling them elsewhere.
Obviously this isn't something that really happens with GOG, because there have been very few bundles where you can get GOG keys from, and in some cases there have been technical solutions to prevent that, like binding your GOG account to another service.

On a darker area, and I'm not sure if this is applicable to GOG, there have been rumours that some developers are selling keys to their own games on some questionable sites. If you ask how and why, well, first they get a number of free (Steam) keys for reviews and other purposes. Then, when they sell them, they get almost 100% profit, cutting off the store as the middleman and possible taxes.

I don't know how widespread this is, but presumably it has happened more than once, because it is a known phenomenon. Presumably any developer who has any real reputation wouldn't do that for a number of reasons though.

what i read this isnt dark at all , steam totally allows it until prices are not below steam prices , and from what i see they don't even enforce this criteria

here: https://partner.steamgames.com/doc/features/keys

Buying keys on eneba is legal.

Buying stolen property (without knowing about its illegal nature) is usually legal too.

Not here. Gets confiscated - as it does in many countries.

To be extremely picky (that's my field), buying stuff you don't know has been stolen is not illegal, it doesn't really get confiscated - you have to give it back as soon as it is known and judicially mandatory, and who sold it to you (the actual person who can be criminally charged) must refund you plus any possible damage or interest that you could have gained after. Only the "fence" gets confiscations.
This is the basic model, with differences from country to country.

In short, you cannot be charged for unknowingly buying stolen goods because you are not a criminal, but you cannot keep it either.

In general, be safe and consider "grey" market as outright black.

it is , i cant see why not to buy at eneba, some scammers can appear probably not more than on any other mass sellers sites , just stick to the high rated ones and it should be fine

depends completely on the laws of the country you are in, there are many different laws on this subject. In the UK you do not get charged for unknowingly buying stolen goods, but you are not the legal owner of the goods and they get confiscated. The mony you have used to buy the goods are lost, unless you manage to sue the seller to get them back.

I see, it's not autmoatic and in two steps. Interesting.
Here the entire process has been "automated" as both the original owner and the new buyer are considered offended parties.

I think it is beacuse the goods can travel trough chains. A fence buys it, sells it to a retailer, the retailer sells it to a customer. Or even longer chains. It is then the customer's onus to sue the retailler, and the retailler to sue the fence. IN this way, the custmer is 'protected' in that it is more likely to sucessfully sue a retaller than the fence. It is also more likely that you never get to the actuall suing part, as most retalilers are happy to do a refund before it gets that far.

Blanket statement. There are legit key stores that sell gog keys at constant discounts. The best example being Gamesplanet, who are legit and authorized by numerous publishers. Not only do they usually have a (at least) 10% discount, but you also get a discount if you're in the "For Uncut" group on Steam (a further 3% just because)

There's a separation between "legal" and "supported" and "unenforced."

Various sites will fall under one or more of those categories. In particular, GOG does not support the resale of gift-codes. This means that, at best, a code at a particular price corresponds to the price of a game on sale. And, odds are, there is likely to be some markup to make profit. However, GOG does not support this, nor does GOG condemn this: you are entirely on your own when engaging in this. However, reselling of keys is indeed legal, however some sites that have illegal activities (pirated games, fraudulent codes, etc) that go "unenforced by law." Whether or not the site you're buying from is legal is the real question. GOG officially does not support resell of gift codes, however they do not prohibit it. They're strictly "hands off," because trying to police this is entirely untenable for them.