The IRS provided guidance for this exact scenario. From https://www.irs.gov/newsroom/irs-virtual-currency-guidance:
Q-3: Must a taxpayer who receives virtual currency as payment for
goods or services include in computing gross income the fair market
value of the virtual currency?
A-3: Yes. A taxpayer who receives
virtual currency as payment for goods or services must, in computing
gross income, include the fair market value of the virtual currency,
measured in U.S. dollars, as of the date that the virtual currency
was received. See Publication 525, Taxable and Nontaxable Income, for
more information on miscellaneous income from exchanges involving
property or services.
Q-4: What is the basis of virtual currency
received as payment for goods or services in Q&A-3?
A-4: The basis of
virtual currency that a taxpayer receives as payment for goods or
services in Q&A-3 is the fair market value of the virtual currency in
U.S. dollars as of the date of receipt. See Publication 551, Basis of
Assets, for more information on the computation of basis when property
is received for goods or services.
In short, for tax purposes, if you recieved 0.1 BTC were worth $1,000 at the time of sale (you would presumably have a dollar valuation for this sale) it will be as if they paid you cash and you immediately bought some BTC at this price. You must track the value and amount of BTC of each individual sale, as each lot will have unique costs basis.
Unless an entity outside the US took possession of the coins on your behalf, this would almost certainly be considered US-based income regardless of where the buyer lived.