re: TD Ameritrade lowers cost per trade to $6.95

If you keep $10K in an IB individual account, and your commissions are less than $10 per month, the activity fee is $10 minus month's commissions.

They will portfolio margin, rather than margin individual stocks.

Various commission structures. You can fix it at .005, minimum of $1 per trade. Or you can tier your structure at potentially lower commissions. Some free ETFs.

But it gets cloudy. You can route trades to get liquidity rebates, etc.

The other rub is you pay for whatever feeds you want. There is some free market data. So you're going to pay for L2 and top of book, and probably need to pay for API access.

You can do a lot of stuff on IB. Contract for differences on anything, if you can find a counterparty. London gold if you can get away with it. Credit default swaps.

But, but, but. They aren't E Trade. They won't give you 3 days on a margin call. Especially if it is portfolio margin, and the risk model falls apart. They will immediately liquidate. You're going to pay for busted trades that aren't your fault.

So if you put $10K there, and make 3 trades a month, you're going to pay $10 that month. Plus news feeds, plus L2, plus some directing charges less rebates. So now you're up to $21 to $31 monthly. But If you just want to pay $10 for (essentially) unlimited trading as a infrequent retail trader, or investor, and don't necessarily want market depth etc., $120 per year is pretty hard to beat. I used the word essentially. I'm assuming low trade volume.

If you have $100K there, one account, or multiple accounts linked, you're done with inactivity fees, can start to get even lower commissions, and would probably buy a monthly bundle of feeds, market depth etc.

As da big fella mentioned, you can short hard to borrow shares, margin is levered with portfolio margin, there is access to all sorts of markets, including some dark pools retail investors can get to, ad infinitium.

I don't trade per se, but what I like to use it for is when I sell off part of my core holdings to take some profit (usually to buy right back into the same security at a better entry point. If I wanted to dump 5000 CVX and WYNN, I'd route the trade to various market centers to not tip my hand that I was unloading. More stable pricing. Some rebates for adding liquidity to certain market centers. There are some other ways you can "hide" trades. They are never truly hidden, but for all practical purposes, they are hidden for the retail guy on TD, and most average people. So you can look at top of book, and see pending buys, and how many shares, and where the buys are coming from. Route 5000 CVX to 6 different places to try to avoid partial fills and what generally drops the price of what you're trying to sell.