[elihu]

Novel life forms and technology could be valuable, especially technology that's hard to replicate (due to proprietary processes/trade secrets or whatnot).

Fissionable materials might be valuable to planets that have none of their own and want to make use of nuclear power.

People are a resource of their own that may be wanted on sparsely inhabited planets (especially if birth rates on technically advanced planets are similar to those in developed nations here on Earth).