The continuance of this relation demands that the owner of the labour-power should sell it only for a definite period, for if he were to sell it rump and stump, once for all, he would be selling himself, converting himself from a free man into a slave, from an owner of a commodity into a commodity. He must constantly look upon his labour-power as his own property, his own commodity, and this he can only do by placing it at the disposal of the buyer temporarily, for a definite period of time. By this means alone can he avoid renouncing his rights of ownership over it. [3]
Marx is talking about the power of labor, and how it is the sole commodity we have as a person. Someone without wealth will exchange themselves at some rate of labor for another commodity. Marx notes the importance of limiting the labor you sell so that you still control ownership of it. It reminds me of the video we watched about the Lawrence strikes. Those workers were selling to much of their own property for nothing, and it was resulting in horrible lives and living conditions