1 Mummy vendor: There's an oversupply of mummies being excavated from ancient tombs.
2 Mummy vendor: The Egyptian Museum can't store them all, so we're selling them to raise money for better facilities.
3 Monty: That's barbaric and outrageous!
4 Mummy vendor: Well, it's better than just pharaohing them out.

The Egyptian Museum in Cairo contains the world's largest collection of Egyptian antiquities.

And there really was an oversupply of mummies excavated from ancient tombs. From the 12th century or so, ancient apothecaries produced a supposedly medicinal substance known as mummia from actual Egyptian mummies. Mummia had been around for centuries, made from natural bitumen deposits. The Arabic word mūmiyā refers to bitumen, and it was transliterated into Latin as mumia, a word used to refer both to medicinal mummia made from bitumen, and also to ancient desiccated corpses preserved by using bitumen as one of the preservative ingredients. Thus this is the origin of the English word mummy to refer to these ancient corpses.

Originally mummia was made just using natural bitumen from deposits in the Middle East, but supplies of this could not keep up with demand around the 12th century, so a new source was found: the residues of bitumen found inside Egyptian mummies, which were at the time being uncovered by the hundreds in Egypt. It's astonishing to think that this became a more reliable supply of bitumen than natural deposits.

Eventually the demand for medicinal mummia became so great that manufacturers decided to simply grind up entire mummies, rather than scraping out the bitumen from inside the corpses. And eventually, by the 16th century, manufacturers decided it was even easier just to find fresh corpses and dry them out in the sun before grinding them up. Mummia produced in this way continued to be used medicinally up to the 17th century, when finally physicians decided that using ground-up dead people as a medicine was probably not such a great idea.

Another use for the abundance of mummies began in the 16th century. Paint manufacturers ground them up and used them to make brown paint. So called mummy brown was a popular rich brown colour among artists from that period onward, particularly favoured by the 19th century Pre-Raphaelite movement. It was only around this time that artists became fully aware of the fact that the paint they were using contained ancient human remains, and this led to its eventual demise, although some manufacturers continued to make mummy brown from actual mummies into the 20th century.

And yes, you really could just go to Egypt and buy a mummy off a guy in the street. Here's a photo of a mummy vendor, taken in 1875 by French photographer Félix Bonfils:

[Public domain image from Wikimedia Commons.]