This is the biggest misconception everyone has: that everyone’s time is equal in value. Yes, you are correct, not everyone’s time is equal in value. Take for example the painter and the lawyer.
The painter and lawyer charge always charge different rates for labor–they have different skillsets to provide and demands from the market. The lawyer certainly may charge much more than a painter does for each hour of labor performed–the laywer has personally spent thousands of hours (and money) learning the knowledge required for the occupation. Similarly, the painter has also spent many hours (but probably not as much money, unless they’re not painting houses but portraits) perfecting the skills and acquiring the tools used for being a painter. They certainly with charge anyone a different rate per hour to perform the work.
The painter might earn ξ3/hour and the lawyer might earn ξ30/hour. The painter still has to work 10 hours “actual” time to earn enough to pay a lawyer for an hour of “actual” time because of skill or ego. When it is money, it is equal–if someone hands you a $50 bill, you never question its value from the occupation that earned it. When it is price, it is not–which is why a painter may perform the work for $50 and a lawyer would at best give you his card. This is why using ξ isn’t barter–you’re not trading painting for laywering, you’re trading time for painting or lawyering in a transaction where both parties agree to a price. It isn’t expected that a painter and lawyer would “trade equally in time” unless the painter and lawyer are “good buddies.”