As long as it's clearly stated before the raid starts, no one is mislead in any way about how loot with be handled, and the only currency used in any "sale" is in-game gold, it isn't really breaking any rules. The "ninja looting" rules exist to protect players (to an extent) from loot being stolen, but nothing is being stolen if they willingly join a group where certain drops will be off-limits.
Understood the part about the rules. Still cant see why would anyone go into such a raid, apart from his/her friends and/or familiy. All the standard rules are being bypassed to benefit the raid leader and turn the raid into P2W.
Not advertised as gdkp run, only certain items for sale, others reserved. No gold split afterwards
You: "Hey, is the gold split after the raid's end?"
Them: "Yes." -> No split -> Ninja
Them: "No." -> No split -> Not ninja
You didn't ask*, and there were no specification of splitting the gold at the end, not a ninja.
*Not asking for specifics implies you understand/know the rules. Ninja rules aren't there to protect you from your own assumptions.
Since it wasn't advertised as gdkp run, I'm inclined to believe (without seeing the exact wording) the leader knew exactly what they were doing.
Their plan was to lead the raid, reserve few items as they always do, and collect the loot and sell the items to highest bidders. Their plan didn't include sharing the profits.