I've found a Japanese site that seems to sell a lot of Japanese MSX cartridges (only the cart) at good prices.

You can find there (amongst many other titles):

1905 - ALESTE - 880 yens
1906 - 1942 (MSX2) - 880 yens
1941 - QUARTH - 480 yens
1920 - F1-SPIRIT - 380 yens
1932 - PENNANT RACE 2 - 380 yens
1982 - PARODIUS - 780 yens
1993 - FANTASY ZONE - 1180 yens
1989 - FIREBIRD (HINOTORI) - 1180 yens
1914 - USAS - 880 yens

I don't know if the seller will understand English though. Take a look to the complete list:
http://homepage2.nifty.com/fctika/msx.htm

Some of them are now sold.

to you?

No.

Can you see if these are only carts, or complete with box and manual? I couldn't find pics...

I believe that they are only the carts (this is what Altavista translation service said if I remember correctly).