This is a fair question to ask, given that it's a fairly common assertion that isn't always understood as well as it is eloquently made. Frequently, the folks discussing it understand the Catholic teaching on indulgences about as well as they understand the Buddhist teaching on the transference of merit, which is to say that they really don't understand it or that they have basic misunderstandings about it.
Fr. Mike gives an excellent explanation of what indulgences are; it's probably the best I've heard from any priest. That said, I have some concerns about what was not said in the video, and I think there are additional distinctions to make that are important.
The way that the question is framed makes his answer technically correct, which as we all know is the best kind of correct. On the other hand, technically correct positions tend to leave out significant information.
It's true, as he says in the video, that the Church does not teach that indulgences are a means of buying grace, and thus does not sell them as an institution because it doesn't believe that they can be sold. This, however, has not stopped unscrupulous clergy from acting as if grace could be bought by way of indulgences. While it's true that the Church officially condemns simony in its various forms and has set severe canonical penalties for it, there are always those who are willing to violate the canons for personal gain.
And because many people see each individual member of the clergy as synonymous with the Church (which is not a theologically or institutionally correct view, but a nonetheless prevalent view), it certainly looks like to them that the Church is selling indulgences. Certainly, I don't see it as any particular fault of theirs that they call it as they see it, except perhaps after it has been effectively explained to them.
I'm not sure that Fr. Mike was avoiding the topic of real instances of simony that caused changes to canon law in the Middle Ages, but it does seem fairly common for Catholic apologists to carefully avoid saying up front that Catholics sometimes did terribly sinful things and still do terribly sinful things. I'm often irritated when Catholic apologists try to downplay the very real sins committed by Catholics in the apologetics process.
Trying to avoid the issue just gives credence to the bad argument that people doing bad things is evidence against the truth of the beliefs held by those people. Talking around the issue of serious sins on the part of clergy is not the right way to do apologetics. I think the right way to do it is to just say that yes, some Catholics did sin in the way they abused indulgences, that it was tragic and terrible, and that the Church still condemns it as a sin.
And what's more, we need to show that we are willing to take concrete steps to reduce the occurrence of the sin on an institutional level. True repentance requires no less than a firm purpose to amend our lives and taking action to amend them, after all.