I recently found a thread here with some sites where you can trade Stocks with only BTC as the main balance without having to convert to USD.
But the problem is: I don't know which one has the lowest fees.
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I also have read that not all results from Google are trustworthy theses days. Have also heard from WorldMarkets.io via Bitcoinist but the liquidity is just not there and the interface is kinda bugged for me.
There was a thread not long ago about such a service:
https://bitcointalk.org/index.php?topic=5097810I don't think that such services are so widely used for now, so you'll need patience in getting reliable feedback.
And you may also have to explain what you meant about the lowest fees.
My guess is that if you want to only sell BTC for USD, you better sell directly (*) and if you want to diversify your (bitcoin) investment into stocks too, then it may be cheaper to use this kind of services.
(*) there's one modifier I don't know though: the taxes. You may get into strange situations in some countries (like mine starting this year) that Bitcoin is unreasonably heavily taxed and you may (at the end of the year) get out cheaper via stocks.