Actually, no he doesn't. Per the 2015 Penguin edition:

On quitting the kingdom of Kinsai and entering that of Fu-chau the traveller journeys south-eastwards for six days through mountains and valleys, passing cities and towns and homesteads in plenty. The people are idolaters subject to the Great Khan and are all under the government of Fu-chau, with which we have now begun to deal. They live by trade and industry and are amply provided with the means of life. There is abundance of game here, both beast and bird, besides lions of great size and ferocity. Ginger and galingale are superabundant: indeed for a Venetian groat you could buy a quantity of fresh ginger equivalent to 80 lb. There is a fruit here that resembles saffron; though it is actually nothing of the sort, it is quite as good as saffron for practical purposes. Furthermore, you must know that the natives eat all sorts of brute beast. They even relish human flesh. They do not touch the flesh of those who have died a natural death; but they all eat the flesh of those who have died of a wound and consider it a delicacy.

The bit about cannibalism is an afterthought inserted near the end of a section on local food, and he does not claim it was sold openly in markets.

Now, there are often manuscript issues around Polo, and it is worth bringing up that this does actually serve to excuse a lot of what Polo gets accused of, especially as later manuscripts tend to be the source of the most egregious claims. For instance, the claim to being at the Siege of Xiangyang, often used as a major 'gotcha' by the anti-Poloists, is of dubious veracity as it appears only in some later manuscripts and is absent from the earliest known ones. However, in this case the claim appears to be present in most of the reliable manuscripts, and so it does need to be treated as basically an original element.

So then the question becomes, is this factoid true? Almost certainly not.

How do we explain it? Well, one of the only bits of English-language scholarship I could find on the topic raises an interesting point, that being that cannibalism was a relatively common late Medieval trope in depictions of the 'exotic' Orient, particularly the further reaches at the edge of the known Orient. Laurence Harf-Lancner, comparing Thomas of Kent's Roman de toute chevalerie (a French Alexander Romance) to Rusticello's Book of Wonders, suggests that Polo was, in this particular instance, repeating a trope contained in other widely-circulating texts about Asia: in particular, the Roman de toute chevalerie, in its fantastical ethnography of India, also includes descriptions of cannibals. Polo's description of cannibalism in Fuzhou, then, may be an intentional appeal to existing motifs. It is worth also noting that Fuzhou, being a major trading harbour, would have been comparatively cosmopolitan and also a site where Polo could claim more exotic things taking place, especially because he also claims that cannibalism took place on Sumatra and the islands of the South China Sea, regions which would have had obvious commercial links to Fuzhou.

Does this impinge on Polo's general reliability? No. As has been stressed by many of the anti-anti-Poloists, we ought not to treat Polo as an all-or-nothing source: he can be reliable in particular areas and unreliable in others. And as Peter Jackson in particular has highlighted, Polo's role in the Great Yuan was as part of a corps of segregated Persian-speaking errand-runners, which meant that in China he travelled a lot but mingled very little. Hence, his descriptions of physical geography and Mongol administration are generally very reliable, but is much shakier on social and cultural matters to which he genuinely had very limited exposure. I discuss this more in this answer.

In short, Polo told a lie. He does that sometimes. But he doesn't lie all the time, and the critical thing is that there are clear patterns to it that allow us to make use of Polo as a source regardless of the places where he is more dubious.