Just put some decent kits together, create PDF/video guides on how to install Retropie, how to put ROMs on the system and all the other basics.
Don't forget to use homebrews for examples and mention that using ROMs of games you don't own, or BIOS of systems you don't own is illegal.
Give credit to RetroPie/homebrew developers.

Doing this I think you are in the clear.

@sfc_k Mine arrived in the end and I'm very happy with the product, hopefully yours turns up soon :)