It’s hard to answer that without actually seeing a modern gift economy in action. Luckily, modern gift economies actually do exist. On a small scale, they exist among friends, who might lend each other a vacuum or a cup of flour. There’s even an example of a gift economy on a much larger scale, albeit one that’s not always in operation: The Rainbow Gathering, an annual festival in which about 10,000 people gather for a month in the woods (it rotates among various national forests around the country each year) and agree not to bring any money. Groups of attendees set up “kitchens,” in which they prepare and serve food for thousands of people every day, all for free. Classical economists might guess that people would take advantage of such a system, but, sure enough, everyone is fed, and the people who don’t cook play music, set up trails, teach classes, gather firewood, and perform in plays, among other things.
Other countries though do not have the reporting requirement that the U.S. does concerning proceeds from barter transactions, but taxation is handled the same way as a cash transaction. If one barters for a profit, one pays the appropriate tax; if one generates a loss in the transaction, they have a loss. Bartering for business is also taxed accordingly as business income or business expense. Many barter exchanges require that one register as a business.
It was the fifth lasagna that did it. Three months into the project, Simmons had signed up over 80 Barter Babes, and every time she bartered her financial services, she seemed to get a lasagna in return. She was trying to shove yet another five-pound pasta dish into her freezer when the tears started to flow. “I don’t need another lasagna!” she yelled at her boyfriend. “I need a haircut! I need a bicycle!” She was overwhelmed with doubts about her bartering experiment: I’m so ridiculous. What the hell was I thinking? This whole thing is never going to work. Trades weren’t happening at the frequency she had anticipated, and Simmons was feeling depressed. She couldn’t afford a dye job at a salon, and she had started gaining weight. She’d gone from ballin’ to having $200 in her joint account.
Another advantage of bartering is that you do not have to part with material items. Instead, you can offer a service in exchange for an item. For instance, if your friend has a skateboard that you want and their bicycle needs work, if you are good at fixing things, you can offer to fix their bike in exchange for the skateboard. With bartering two parties can get something they want or need from each other without having to spend any money.
The Owenite socialists in Britain and the United States in the 1830s were the first to attempt to organize barter exchanges. Owenism developed a "theory of equitable exchange" as a critique of the exploitative wage relationship between capitalist and labourer, by which all profit accrued to the capitalist. To counteract the uneven playing field between employers and employed, they proposed "schemes of labour notes based on labour time, thus institutionalizing Owen's demand that human labour, not money, be made the standard of value." This alternate currency eliminated price variability between markets, as well as the role of merchants who bought low and sold high. The system arose in a period where paper currency was an innovation. Paper currency was an I.O.U. circulated by a bank (a promise to pay, not a payment in itself). Both merchants and an unstable paper currency created difficulties for direct producers.
As Orlove noted, barter may occur in commercial economies, usually during periods of monetary crisis. During such a crisis, currency may be in short supply, or highly devalued through hyperinflation. In such cases, money ceases to be the universal medium of exchange or standard of value. Money may be in such short supply that it becomes an item of barter itself rather than the means of exchange. Barter may also occur when people cannot afford to keep money (as when hyperinflation quickly devalues it).