Helping each other with no set amount or expectation of exchange is simply that -being neighborly and helpful.
Setting an exchange rate makes it actual "barter" and, as such, is subject to taxation. If the IRS finds that you are bartering your goods, they will expect you to report the exchange as "income" and pay taxes on that amount.
Back in the 1970's there were quite a number of "barter exchange" groups/organizations/co-ops that started to grow because people thought it was a way to avoid the taxes. The IRS stepped in and started auditing taxes of people who belonged to these barter exchanges and charged interest and penalties on all the income that had not been declared because those bartering did not think it was income.
Barter Exchanges disappeared about as fast as they started - and I have not seen them come back since.