I think three types of domain may (note emphasis) become valuable under .jp:-
A) Easy to spell Japanese domains. By this, I mean domains where there's no potential for argument about how they could be spelled. If you know Japanese, you'll understand that you can make a case for writing "tu" or "tsu", "oo" or "ou" or "o-", etc. So it's better to go for something obvious, short, easy-to-remember and which can't be misspelled - for example kuruma.jp (car)
Remember too that Japanese people aren't as used to typing "Western" (or in this case Romaji) letters as we are, so beyond about 9-10 letters the domain's getting *too long* for the target market.
B) Foreign terms that have been absorbed into Japanese via Katakana, if they're VERY generic and widely known. Example: loan.jp, home.jp... Same qualification as above re. length etc. Also, plurals are pretty much a no-no as the concept doesn't exist in Japanese so for example loans.jp might only be worth 5-10% of loan.jp, even though in the .com market loans.com might be worth MORE than loan.com!
C) Very very very generic English-language domains (i.e. "Top 100" quality names). Things like weather.jp for example. And/or domains that would have a strong enough market just from the (relatively small) foreign community here.
Unfortunately I don't see any potential for a domain like sold.jp. It doesn't work in Japanese in the sense that people aren't (generally - you can always find exceptions) familiar with the past tense of "to sell". Now if you'd had sale.jp, that would probably worth a bit because people would instantly recognize it from "se-ru" (Katakana) which is displayed in just about every shop and department store at one time or another.
One final thought: Japan is probably around 2-3 years behind the US and at least 18 months behind Europe. If you look at the near-dead market for .co.uk or .de domains at the beginning of 2003 and earlier, it probably gives you some idea of the current situation.
Couple that with the intensely difficult transfer process and the current lower penetration of the Web in Japan and any investment in super-premium .jp domains is probably going to take at least 2-3 years to blossom, and more likely 5.
I'm a believer - I have a couple of dozen domains lined up including advertising.jp, leasing.jp, p**no.jp (adult term), yakitori.jp - but I don't realistically expect to see a yen of profit until 2006 or later.