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Strawman purchase is you buying a gun for someone else, with their money.
You buying a gun for someone else with your money, and giving it to them is a gift, and is legal.
You buying a gun for someone you know cannot legally own one is a crime.

And the one that comes closest to what you are asking about, buying a gun, and selling it later, would be "engaging in the business of dealing in firearms" without a FFL (Federal Firearms License). There is no clear cut definiton for this one, and in rare cases, people have been prosecuted for a single gun (but usually unsecessfully). The key is the intent. If you buy a gun with the intent of selling it you may be "engaging in the business". However, with a single gun, or even a few, this is a very difficult thing to proove in court, and generally ignored by the authorities. However, if you do this 30 times a year, and make income from it, that is something else entirely.

You could have a collection of a hundred guns, and sell them all and not be "engaging in the business"

If you bought a new rifle at a gun show, and on the way home your truck blows its engine, and you need to sell the rifle, there's no legal reason not to sell the rifle. If it won't hit the broad side of a barn, and you are so tickked you sell it the next day, same thing. But, if you buy a gun, with the intent of selling it to make a profit, as a means of earning income, without a license, you are breaking the law.

You can sell a gun for a profit, its still allowed. But doing so as a means of earning money isn't. Buy, sell, trade all you want, even make some money, but don't be doing it as a "business" without getting the legal required licenses
(and paying the taxes!)