Ebay is an interesting ecosystem to look at. The volume there is far lower than Amazon, but it does offer some interesting insight.
For example the link you provided is from a seller with relatively few purchase feedback (214 feedback left, 97.7% positive). People hesitate to buy from users without a long history, so they need to sell product at lower prices than their competitors and even then they may not sell as many. That one has thus far sold 2 with 5 still available.
If we look at other ones.
http://www.ebay.com/itm/SEALED-Nint...787967&hash=item25d7cf1879:g:ocEAAOSwhvFZHftU
378.80 USD. 15 sold in the past 24 hours. That seller has a substantial feedback history (4049 feedbacks at 100% positive). People are willing to pay more on Ebay for what they view as a reliable seller.
http://www.ebay.com/itm/Nintendo-Sw...787967&hash=item33d329849e:g:zzAAAOSwjvJZUU-L
384.99 USD. 26 sold in the past 24 hours. That sellers doesn't have as much of a feedback history (648) but at least has 100% positive ratings from a decent pool of purchasers.
http://www.ebay.com/itm/NEW-Nintend...876898&hash=item2382a7376a:g:-OEAAOSw3ZRY9mTx
That's a good one for low price and high reputation. 211 sold. 359.80 USD.
Here's the point. Every single one is still quite a bit above MSRP.
Even on E-bay people are willing to pay up to 380+ USD in order to obtain a Nintendo Switch that has a retail MSRP of 299 USD.
That shows that demand is very high. Also, note that while you mention that the Switch is doing well in Japan, you fail to mention that the Switch is selling double that in NA and almost double that in Europe. Using data to be taken with a grain of salt from VGChartz (51,468 NA, 43,478 Europe, 24,821 Japan for the week of June 03, 2017)
Using your predictive numbers for Japan (12-20 million, I don't know what you base those off of) that would mean that it would be ~60-100 million for Japan, Europe and NA. WW it would be higher as that doesn't account for ROTW (rest of the world). Or in other words far more than either the PS3 or X360. If you think the Switch is going to do badly, you might want to revise your guess for Japan.
Regards,
SB