Sounds like you may need to speak to a better tax professional in your area.
I am not now and have never been a tax professional and I live in the United States. I also do not use online crowd sourcing sites to earn an income.
TL;DR: You pay taxes where you live or your business is located, not where clients may be located.
The purpose of taxes is to support infrastructure in your location, not in some other country. Taxes assist in building roads, paying for schools, paying for government in general. Since you don't benefit from any of that in another country it's not customary to pay taxes in another country unless you are based or live in that country. You don't pay taxes in the U.S. unless you live in the US or have a business location established in the US.
One caveat may be goods or products. If you are importing/exporting items from/to the US, then there may be tariffs required, but tariffs are not income tax.
For example, I live in the US, my work is all ephemeral (digital in nature, no goods or products), there has never been a need for me to pay any UK taxes even though I've had UK clients. I'm not earning any income inside the UK.
In addition, think about it.. if you don't live in the US and don't pay US taxes, what possible penalties could the US impose upon you? The answer is none.
You may owe taxes in your location, wherever that may be. You may be earning income from the US but live in CountryX. Most likely that income is taxable by CountryX since that is your location. Those tax filings in CountryX would be your tax records in lieu of any US tax records.
Further example... even though I don't file UK taxes when I have UK clients, I still must claim the income I earned from UK clients on my US tax returns... and pay US taxes on the income I earned.
As for specific web sites.. you'll have to go over their terms of service. They could have specific terms related to non-US users. Certainly informing them that you aren't a US citizen or business may alleviate the tax issue. Of course, there could be other repercussions if they don't permit non-US transactions.
Whether or not using a US-based crowd-sourcing web site constitutes a "business location: inside the US.. again, you need to speak to a qualified tax professional.
I would stress again, speaking to a qualified tax professional in your location who can provide a definitive answer may assist you. Get it in writing so you can hold them accountable if thy are incorrect. If they know the answer, it should be no issue for them to put it in writing.
In short, you pay taxes where YOU live, not where your clients live.