Just so you know, I've done a fair amount of business with this gent and never had a single problem. Shit can happen, but his feedback score speaks for itself. In doubt, PM him before jumping on a 1 cent auction.
As for the Save-kun, indeed it's a Memory Base 128 clone.
IIRC, Save Kun was most similar to MB128, because it only works with specific games (i.e. Brandish), unlike the Backup Booster /Tennokoe Bank 2 (which work universally).
A useful precision here: the Memory Base is not an external back-up unit like the Back-Up Boosters or the Ten no Koe 2, it's a storage device like the Ten no Koe Bank. It works exactly like a TnKB actually, except it has 128 kB instead of 8. You can store up to 64 back-up RAMs instead of 4.
The only difference between the TnKB and the MB128 is that the TnKB has its own built-in bank management utility (which starts when you power up the console with the card inserted), while the MB128 needs a CD game with a dedicated (and often hidden) MB128 utility, such as Linda Cube, Emerald Dragon and a couple others.
Moreover, I seem to remember that a handful of games allow you to directly save onto the MB128. I'm not positively certain of how it works, but I believe Vasteel 2 is one of them.