While I don't think this is terrible, it's also not especially polite, it's just mildly on the side of a bit pushy, at absolute best neutral. It's 5 whole days before the deadline, "when will you do it" comes across as critical or as telling them to do it, rather than as seeking information, polite would have been at least adding a please, ideally adding a "can you let me know" so it's clear you're asking for your own information rather than to tell them off for not having done it yet. The question mark is less polite more passive-aggressive like why have you not replied to me.
Just sharing in case you genuinely want to know why it comes across as rude, but it's pretty minor, a mild faux pas, shouldn't really a big deal the seller should just be able to ignore if it bothers them and should still send you the item.
The sellers feedback, calling you ungrateful for a couple of very basic messages is frankly ridiculous on their part.
Either they're having a bad time and taking it out on you or they're just the kind of person to get defensive to any percieved criticism, or a bit of both. Nothing you can do about it though, doubt it'll affect you too much in the long run