Yes. .de Domains are no problem to sell / transfer. The new owner must just take care about, that he has a person (mostly Admin-C) who is based in germany - rule from NIC.
Ok, then GoDaddy integrated there the .de system may be not completely. .de is also not their core business.
May be you should look to register your .de domains in future with for example www.dd24.net, they have a good system therefore and offer teh whole site also in english.