The selling thing is a nice midrash. And indeed those are valid questions if we go with the approach of the midrash.
One alternative, however, would be a different reading of how the plague worked. This escapes your questions, and it follows the simplest reading of the actual verses.
Rabbi Aryeh Kaplan's translation:
7:19 God said to Moses, 'Tell Aaron to take his staff and extend his hand over the waters of Egypt - over their rivers, their canals, their reservoirs, and every place where water is kept - and [the water] shall turn into blood. There will be blood throughout all Egypt, even in wooden [barrels] and stone [jars].'
7:20 Moses and Aaron did exactly as God had instructed. [Aaron] held the staff up, and then struck the Nile's water in the presence of Pharaoh and his officials. The Nile's water was transformed into blood.
7:21 The fish in the Nile died, and the river became so polluted that the Egyptians were no longer able to drink the Nile's water. There was blood everywhere in Egypt.
7:22 However, when the master symbolists of Egypt were able to produce the same effect with their hidden arts, Pharaoh became obstinate. He would not pay attention to [Moses and Aaron], just as God had predicted.
7:23 Pharaoh turned his back to them and went to his palace. Even to this [miracle], he would not pay attention.
7:24 The Egyptians dug around the Nile for drinking water, since they could not drink any water from the river.
I think the simple reading is that it affected any stored water, or any open bodies of water. But the Egyptians were able to dig for subterranean water.