Domains that appear particularly lucrative are registered by traders and linked to a simple website that informs users that the rights to this domain can be purchased as part of a domain trade. Domains that are relatively well listed on search engine results pages (search engine optimization, SEO) are particularly sought after in domain trading.
Further references:
https://de.wikipedia.org/wiki/Domainhandel
http://domain-recht.de/domain-handel/goldene-regeln-fur-den-domain-hand…
The following webinar provides an introduction to domain trading: