This is the relevant section of the Terms of Use:
You agree not to use the Platform, the iNaturalist Services, or any iNaturalist Content for any illegal, unlawful, or unauthorized purpose or activity, including but not limited to threatening, abusing, soliciting, spam, harassing, stalking, impersonating, or intimidating other iNaturalist Users.
Asking someone for specimens can be OK, but the person should be able to discuss how they’re doing this legitimately and legally. If it’s unlawful, they should not be using iNaturalist for this. Personally I would not send someone anything unless they bring up legal issues and how they’re complying with the law (“I can provide you with the proper permits for sending a specimen across state/country lines”) and also send me an email address linked to an institution like a university. IMO their profile should be filled out as well. Yes, that can be faked, but it’s another way to build trust and be transparent.
Are these via messages? If so, please flag these messages if they’re suspicious so staff can take a look.