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.
The Buy-day (Wheat) Ecological Life Associate summarizes a vision of life in Gezi as follows: "In our world, which is being poisoned and destroyed by consumer culture, we need sustainable and self-operating models of lifestyles, including a barter economy, ecological food production, arts and craftsmanship based on needs, renewable and effective energy use, agricultural models backed by society, permaculture, slow cities, transitional towns, eco-villages, district gardens and secondhand and recycling systems.
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 IOU 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.
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.
On the positive side, there are great advantages to bartering. As mentioned earlier, you do not need money to barter. Another advantage is that there is flexibility in bartering. For instance, related products can be traded such as portable tablets in exchange for laptops. Or, items that are completely different can be traded such as lawn mowers for televisions. Homes can now be exchanged when people are traveling, which can save both parties money. For instance, if your parents have friends in another state and they need somewhere to stay while on a family vacation, their friends may trade their home for a week or so in exchange for your parents allowing them to use your home.
An alternate currency, denominated in labour time, would prevent profit taking by middlemen; all goods exchanged would be priced only in terms of the amount of labour that went into them as expressed in the maxim 'Cost the limit of price'. It became the basis of exchanges in London, and in America, where the idea was implemented at the New Harmony communal settlement by Josiah Warren in 1826, and in his Cincinnati 'Time store' in 1827. Warren ideas were adopted by other Owenites and currency reformers, even though the labour exchanges were relatively short lived.
Trade did occur in non-monetary societies, but not among fellow villagers. Instead, it was used almost exclusively with strangers, or even enemies, where it was often accompanied by complex rituals involving trade, dance, feasting, mock fighting, or sex—and sometimes all of them intertwined. Take the indigenous Gunwinggu people of Australia, as observed by the anthropologist Ronald Berndt in the 1940s:
Whether or not one agrees with such broad claims, it’s worth noting that monetary debt, a byproduct of currency, has regularly been used to by some groups to manipulate others. Thomas Jefferson, for instance, suggested that the government encourage Native Americans to purchase goods on credit so they’d fall into debt and be forced to sell their lands. Today, black neighborhoods are disproportionately plagued by debt-collection lawsuits. Even after taking income into account, debt collection suits are twice as common in black neighborhoods as in white ones. $34 million was seized from residents of St. Louis’ mostly black neighborhoods in suits filed between 2008 and 2012, much of which was seized from debtors’ paychecks. In Jennings, a St. Louis suburb, there was one suit for every four residents during those years.
Anthropologists have argued, in contrast, "that when something resembling barter does occur in stateless societies it is almost always between strangers, people who would otherwise be enemies." Barter occurred between strangers, not fellow villagers, and hence cannot be used to naturalisticly explain the origin of money without the state. Since most people engaged in trade knew each other, exchange was fostered through the extension of credit. Marcel Mauss, author of 'The Gift', argued that the first economic contracts were to not act in one's economic self-interest, and that before money, exchange was fostered through the processes of reciprocity and redistribution, not barter. Everyday exchange relations in such societies are characterized by generalized reciprocity, or a non-calculative familial "communism" where each takes according to their needs, and gives as they have.