No, you don't need the multitap to use the twinstick -- the guy was just bundling that in his auction, for whatever reason.
As for the cost of the twinsticks, I'd say the saturn VO is perfectly playable using the joypad, so I personally wouldn't spend more than $20 for it. But if you ever wanna play the much better sequel on the DC, it is absolutely mandatory, and don't let anyone else tell you otherwise. Just buy the total control 3 adapter SS->DC controller adapter, and you'll be able to use the SS twinsticks on the DC, so you're golden.
All that said, if you ever visit Asia, Saturn Twinsticks are RIDICULOUSLY cheap.
I found:
- ~$20 (or was it $30? Whatever...) for 2 brand new, boxed twinsticks, with a shrink-wrapped copy of Samsung release of Virtual On. (in 2000)
- ~$3 used, was missing one truss screw on top, but it was in 100% working order. (in 2005)
Granted, those were still bargains in each country (Korea and Japan, respectively), but even full-pop prices are still waaaaay less than what they'd be in the west.