you can eat eggs until they float if you put them in a glass 0f water. if they are upright rather than lying flat make sure you cook them all the way through. I would not sell any older than 10 days.
Ipersonally have eaten eggs quite a bit older than 3 weeks (tested with the method) and have never been ill. Duck or hen eggs, same rules for me, although we are supposed to cook duck eggs all the way through and also tell people who buy them. I tell but eat them whichever way I want them myself
.