Why did the Church sell more indulgences during the Renaissance?

The Church never did. "Pardoners"who often were not even priests did, along with a few bad priests, such as the Dominican monk named Johann Tetzel. Tetzel is reported to have been the inspiration for Martin Luther's 95 Thesis.

The sale of a Spiritual thing is known as Simony, after Simon from The Bible. See the related link. Find out more about the sin of simony in the related link.

To have sold a indulgence is a unspeakably grave mortal sin, one of the worst spiritual sins one could ever commit, and the Church never has this acknowledged this as an acceptable practice.

It's a classic mistake (or rather a Protestant myth) of assuming corruption within theCchurch, was corruption of the Church.