One problem is that the government keeps track of what you sell, like any other business. So you have to keep complex and complete records.
Plus, make sure you charge enough for shipping. Some things are going to have to be packed.
Shipping overseas costs more.
Some countries have great post offices; India, for example.
Others, like Italy and Ireland, are not so good. (Some people actually refuse to sell there, as a result.)
Be sure your product description is complete and lists any flaws, so that the buyer doesn’t want a refund.
Shipping is a killer,learned that real quick;)
I experienced "winning" bidders who didn't pay up, I bought from people who changed the deal when it was time to complete the transaction, etc.
The clincher was when I sold and shipped a $250.00 item that the buyer claimed he never received. I made the mistake of not insuring the delivery.
I asked the buyer to be patient, that I had, indeed mailed it and that it must be delayed somewhere; but the response was not only rude but increasingly threatening, demanding a refund, which I finally reluctantly sent.
A day later eBay advised me via email that the purchaser's account had been hijacked, and I had been scammed out of $250.00+product.
eBay offered no further assistance.
No, the government expects you to tell them what your profit is. But yes, the records keeping is a must. But that's not all that complex, you just need to keep a record of everything you sell, what you paid for it, the postage you pay, and the fees you are charged. Do it regularly, and it isn't that hard. A spreadsheet is recommended.
Apart from what you report to them, the big thing for active sellers is the 1099K that PayPal generates which reports your total receipts for the year to the IRS. This only happens if you take in more than $20K AND more than 200 sales. It must be both, and is based on the total amount of money processed by PayPal, with is higher than the total sales amount because it includes shipping that the buyer pays.
International shipping can be problematical. The eBay Global Shipping Program works quite well, and helps you avoid many of the potential hassles dealing with international sales. The big advantages are that you ship the item to the processing center in the US, and once delivered there, you have no liability for non-delivery. Also, they use shipping methods that always (?) have delivery confirmation, which USPS doesn't for many countries (unless you use Express mail, which is very expensive).