Should it be 'those' shoes?
If you point out 그 shoes, you better say those shoes.
I'm not sure but I don't think so, because you aren't talking about some specific ones here but about "the shoes" in general.
for these types of sentences 판매할 거예요 sounds more better
Duolingo accepted my answer even if it's not in order :-)
The use of 팔 should also be accepted.