Wealth? Vast differences in wealth are not necessarily producing exploitative employment relations - social status differences do. Service workers earn less because of their lowly social status (as feminists and segmented labor market folks have argued) rather than because of the quality of their work or a wealth difference between vendor and emptor.
Restaurants are a case in point. Waiting on tables in a greasy spoon joint is paid considerably less than waiting on tables in a 5-star restaurant, even though the waiter/patron wealth differential is much higher in the 5-star restaurant than in the greasy spoon joint. What makes the difference is the social prestige of these two places - which explains not only wage differences but also who will work where.
PS. I do not buy Kelley's argument that service employment relations spoil social solidarity either. Labor unionism suggests to the contrary. What is more, Michael is absolutely correct that replacing cash relations with social/kin relations can be extremely exploitative (c.f. patriarchy).
Wojtek