If your German is weak to nonexistant, the way to go is to write a message in English. Use very simple sentences. Don't use any slang. Run it through one (or more) of the translation services (Babelfish, Google, Alta Vista). Then run the result back the other way and see how it looks. Both translations will introduce errors, but it the retranslated version is way off the tracks, you should consider rephrasing again.
(One problem is that we have many words that have multiple distinct meanings. So do the Germans. The problem is that they're not the same. For example, in English nut is both something to eat and one half of a fastener. In German, there are two words, Nuss and Mutter, respectively. But Mutter also means mother.)
Anyway, I'd send an email with the original English version first and then the translated German version. Between the two the seller may be able to figure out what you're asking.
I know someone who bought a bike on ebay, paid to have it shipped, then was so disappointed in the bike it just sat in the corner of his garage for six years. He finally sold it for $1,500 less than he paid for it six years ago. That's the bad part of the story. The good part is.... he sold that 1957 R-50 to me!!
I was sort of interested in the bike and wondered a few things about it but couldn't contact the seller. Of course, things being in German doesn't help either...
Kurt in S.A.