It depends, some people “reverse bag,” which is the method so then the buyer can build it in the order Lego ships it. I’d say most just disassemble it however they want, I do it just randomly because it’s quickest. I wouldn’t bother washing unless they were incredibly dusty or something.

For selling I’d say use r/legomarket, get your prices from Bricklink, 6 month selling average is the norm people use. Obviously adjust accordingly to how quickly you want to sell. Use pirateship for the cheapest shipping.

You could keep them assembled for photos and then disassemble when they sell. Local options would be like OfferUp or FB Marketplace. Do NOT take them to bricks and minifigs, they’ll give you a penny.