You can start as small as $2-3 by trading small alts and flip them for substantial profits.
The only thing you should keep in mind while dealing/trading in cryptocurrencies is that you should bet only what you can afford to lose.

Good luck!

I sell them either offline or on platforms like localbitcoins or coin-market to get a deal done via payment methods convenient to me.

I would say it depends upon the volume of the coin.
Also, few coins follow some progressions that you can look for and execute decent trades.

ICOs are speculative but in case the project seems promising then its worth investing into them.
So before you make any movement do thorough research on the project and their roadmap and check what the community has to say on the project.

Good luck!