Hi -- The "Büergel Wirtschaftsinformationen" is the German equivalent of our companies house, click the link then the Union jack flag to get contact details in English.
http://www.buergel.de/index.php
Your other precautionary measure should be to pay for the item using a credit card if at all possible, you even have protection for the full amount where you have paid a deposit as long as the deposit is over £100 up to £30k. This protection applies and extends to overseas purchases, under the consumer credit act 1974 in the same manner as an item bought in the UK, in the event no delivery or refund is made or the firm went bankrupt.
I am intrigued as to what you're intending to buy, if you feel like disclosing. ?