The American chestnuts (Castanea dentata and C. pumila ) were nearly wiped out by chestnut blight. This is probably what you've read about. Chestnut blight is a fungus that spread from Asia to Europe and America. Asian chestnut species (C. mollissima, C. henryi, C. seguinii, C crenata) are resistant to the blight. European chestnuts (C. sativa) are somewhat susceptible, but not as much as American chestnuts. There are still some remaining American chestnuts "in the wild" but they are mostly killed off as soon as they get large enough for the blight to affect them. Some of them persist and grow back from the roots, only for the trunk to be killed off by the blight after a few years. A very few, rare individual trees survive long enough to make seeds. Some people grow those seeds, in the hope that eventually one of them will turn out to be resistant to the blight. You probably don't want to buy any chestnuts grown from wild-harvested American chestnut seeds. But, it's unlikely that your store is selling this kind of tree.
There are blight-resistant chestnut trees available. The Asian chestnut species are naturally resistant. There are also blight-resistant hybrids and genetically engineered cultivars.
The trees at your local store are probably a blight-resistant variety, but don't assume that. Find out for sure. Read the labels on the trees, they should say what species and variety it is. The label may also say whether they are blight resistant, but if not you can look it up once you know the species and variety. If the label doesn't say, ask.
There are lots legitimate of reasons they may be discounted. Perhaps the grower planted too many trees, and demand hasn't met their supply, so they're trying to get rid of them and plant something more popular. They may want to get rid of them quickly so they don't have to take care of them over the winter. If the trees are a blight-resistant variety of American chestnut, they may even be subsidized. Chestnut trees used to be a major component of North American forests, and there's a lot of interest in restoring them now that we have blight-resistant varieties available.
If the trees look healthy and the seller confirms they're blight-resistant, my recommendation is go for it. Chestnuts are delicious. Spiky though. Metal barbeque tongs will be useful for harvesting.
Note: Make sure you know whether the trees you buy are true chestnuts (Castanea spp.) or horse chestnuts (Aesculus spp.) Horse chestnuts are not edible. In fact, they are moderately toxic. They're perfectly lovely trees, with attractive flowers. Bees love them. So I'm not recommending against buying a horse chestnut, but if you plant horse chestnuts and true chestnuts be sure to learn to tell the nuts apart. It's quite easy to do once you learn. Horse chestnuts come in a husk with short, non-painful spikes. There's one nut per husk, and the nut is rounded on all sides. True chestnuts come in a husk with long, needle-sharp, painful spikes. There's usually two or more nuts in each husk, and each nut has a flat side where it pressed up against its neighbor inside the husk. This flat side/no flat side distinction is not 100% accurate - sometimes a true chestnut will grow one nut in the hull, and sometimes a horsechestnut will grow multiples in the same hull. But it's a decent rule of thumb, and once you get familiar with your particular varieties of chestnut and horse chestnut you will see other differences like color, luster, and the size and shape of the white spot on the shell.