Etrade lets you buy and sell triple zero subpennies as much as you want.
The only downsides are that you cannot trade:
1) a halted stock for the typical 10 day halt period (no one can) but you can trade the ticker after the halt even though it has become a gray market stock,
2) a stock with a DTC freeze (or at least you usually cannot),
3) any stock that has a forward or reverse split UNTIL E-TRADE HAS RECEIVED THE NEW STOCK CERTIFICATES FROM THE TRANSFER AGENT (and that usually takes 10 business days for an OTC Pink).