From the eBay listing:
"The Multari is a portable Atari 2600 with 32 built-in games.
It features a vacuum-formed styrene plastic case, 5" LCD, and no big cartridges to carry around.
This is a portable based off the Atari Flashback 2. It was modded to be able to use game cartridges, then a custom chip with the 32 built in games. The case is vacuum formed. This means the plastic was heated and suctioned over a mold, then it was cut out and used as the case."
And later on it says "Uses an EEPROM to boot the games"
This, coupled with the pictures of the guts of the system, indicate to me that there is no way to use a cartridge with it, to change the games you would need to reprogram the EEPROM.