"Sweating" pipes and plumbing fixtures in summer time or during seasonal changes are not a sign of faulty plumbing. Due to condensation of water vapor in the air, beads of moisture will form in warm weather on any exposed pipes and fixtures containing cold water.
Normally, when not in use, the water and fixture will warm rapidly to room temperature and the condensation will stop. When a toilet tank or other fixture continues to "sweat" for hours after it has been used, it is a sign that cold water is continuing to flow through it, possibly due to an improper adjustment of the tank valve or a leak. In this case, a plumber should be called for checking and servicing. Sweating pipes can be wrapped with an insulation material, which prevents the condensation and formation of moisture.