Just to jump in, I think that's the case, that TD won't allow shorting of stocks below $3....so my 250% recollection as the margin requirement to short a "penny stock" may in fact be correct, it may just depend on the defintion of "penny".

I know one of my brokers, it will only allow shorting of stocks listed on the TSX Composite....

Here's some reading if anyone is intersted:

https://www.investopedia.com/ask/answers/05/shortmarginrequirements.asp