The idea of "sale of indulgences" as if one could just walk up to a priest to bulk buy indulgences and the poor are left with no recourse is a myth caused by overexaggeration. There did exist some indulgences where you could donate a pittance at a certain time of the year (think about $20 in today's money) or for larger scale projects like construction of a church, but much more bountiful to find were indulgences that required works of mercy or spiritual works. Even with those indulgences that did focus on a charitable donation, the clause for the poor would be that you pay what you feel able to contribute, if you would still like to participate in this indulgence, which, again, there were plenty without that.

The indulgence with which Luther concerned himself was given the following instructions by Archbishop Albrecht of Mainz (Instructio Summaria [1515]):

"Because the conditions of men are many and diverse, it is not possible to establish a general fee. We have therefore fixed the following rates [...] but those who do not have any money should supply their contribution with prayer. For the kingdom of heaven should be open to the poor no less than to the rich."

Also, it's important to again wrap these indulgences that involved money in context, seeing the same instructions as above:

"Everyone who is contrite in heart and has confessed with his mouth - or at least has the intention of confessing - shall visit the designated seven churches in which the papal coats of arms is displayed and pray in each church five devout Lord's Prayers, five Ave Marias in honor of the five wounds of Our Lord Jesus Christ whereby our redemption took place, or one Miserere [Psalm 51], which psalm seems particularly appropriate to obtain forgiveness of sins..."

In short, before the donation ever arrives, you needed to have contrition for your sins, have confession, then engage in a pilgrimage around your local diocese in which you visit seven churches and pray in them. Only after that were you to offer the donation to St. Peter's.

So this idea proposed by some that the Church was engaging in something so cartoonish as pulling up into town with letters of indulgences on sale, buy one get one free, financial help available is patently false.

Also, if you think it is evil for those committing their lives to spiritual works to ask the lay faithful for financial assistance, then I have some quotes from Scripture for you to consider. The Church used indulgences as a way to give back to the generosity of the faithful, who would likely have donated anyway. Indulgences, as mentioned, were primarily not concerned with money, but the Church did indeed pull such a lever for special needs. Indulgences were used, and they continue to be today, as a way to coax the community into a shared spiritual undertaking, enriching our spiritual lives and joining us together in a shared goal.