Can I borrow your girlfriend on margin to short her?
Mmmm, 100x leverage.
Pro tip: Go long

If your girlfriend has sufficient market depth, I will make her have liquidity. For the power of my scalar private key is a discrete log. I do point multiplication with her elliptic curves.
Do I retain custody of the magic beans?
Only if I - physically - do the escrow.

And custody of the girlfriend? If she’s of a cryptokinky bent, get her into the blockchain per consensus rules.
If I am present in a thread, the probability of that thread turning to Ciphersex™ converges on 1.