The resellability of a SAFE depends on the terms & conditions specified in the agreement. Usually SAFEs have restrictions on transferability, meaning they cannot be resold or transferred to another party without the company's consent.
Also even if the SAFE allows transferability, it might be sold to Accredited Investors only (at least in the US).
It's a very bad idea for a startup to allow SAFE transferability, as you will not know who are your investors. And it will become a hell to locate all of them once you'll do a priced round.
There was some over-the-conter trading in Telegram ICO SAFTs[1] though.
NOTE: IANAL
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[1] SAFT - Simple Agreement For Future Tokens
startups can have zero value and until there is substantial IP there or customer base it remains zero
so i don’t see how SAFEs can become futures contracts