“FINRA Rule 4560 requires FINRA member firms to report their short positions in all over-the- counter (“OTC”) equity securities to FINRA”.
Market makers short and if price goes up they short even more from the spread-accumulation. A simple google search for available securities (product-financed securities) reflects if broker allows.
Citadel Securities Paying $22 Million for Misleading Clients About Pricing Trades