Is buying fish from ships before the auction considered "receiving the riders" (talaqqi al-rukbān) which is prohibited? And does this prohibition entail the invalidity or validity of the sale? What is the ruling if it is with the seller's consent and knowledge of the prices?

If the boat owners are not residents of the city and do not know the market prices, then the prohibition against meeting riders applies to them, because it is likely to lead to their being cheated due to their ignorance of prices.

However, if the boat owners are market regulars who are aware of its prices, then there is no harm in buying fish from them before it reaches the market or auction.

Regarding the validity of the sale despite the prohibition, the sale is valid but forbidden.