You said that "Selling more than 537.500 shares would require a call. " So could I sell more than that number or that is the max? Just to clarify here...TIA!
537.500 would be the max for online orders (as bigger orders would get rejected) but for phone orders there is no limit. Just be aware of the NSCC illiquid deposit requirement the trade causes, in order to avoid a buy-back by Penson if it exceeds $50,000.