The assets here on OGA that indicate a CC0 license do not forbid reselling. You should probably make contributions to add value if you plan to resell assets found here though. Just scraping the site to flip free assets for a quick buck is pretty rude, but it doesn't sound like that's your plan.

If the submitter stated the assets on their website cannot be resold, then the assets on their website are NOT cc0 licensed. They may be a separate "premium" version of the assets found here... Or the submitter may simply be mistaken about CC0. If resale is forbidden, then it isn't actually a CC0 license. Assets found on OGA should not be treated as the same as assets that look similar elsewhere. That author's assets on OGA are CC0, regardless of what other sites say about separate copies hosted elsewhere.

Your plan sounds perfectly fine to me. Honestly, you can't be responsible for someone else's misunderstand about licenses, but it's also a good idea to have good relationships with your artists, even if you are well within your rights to do something they may not like. Hopefully you will hear back from this author soon and they'll agree that selling your project containing their derived assets is fine with them.

Don't count on a prompt response, even on response at all. The authors don't owe you support. They've clearly stated the license as CC0 both here and on their website. It was 1.5 years ago, the latest tweet from them was 1.5 years ago, maybe they don't walk among the living anymore (it's a fun time to live in, COVID included). Don't put your hopes and welfare on somebody's response to an e-mail. Choose either to trust, to wait, or to give up on these assets if you don't trust them for some reason.

IMO

If you downloaded the asset from here(OGA) and it was CC0, then you are only bound to CC0, no one can change that. As long as you signed in at the time of download it will give you a credits download file to prove it was CC0.
It's the responsability of the uploader to insure that they license their work correctly. If the uploading author is the real/genuine owner of the work and they have licensed it incorrectly, then that mistake is theirs not the end user. If its not a genuine owner then all bets are off and the asset will flagged until it can be sorted out.

The real/geniune owner is entitled to change the license at anytime should they realise thier mistake, but will have to concede that any of the asset downloaded before the change will remain on whatever license the agreed to previously.

That said, the Author/uploader should be respectfully treated, especially when allowing assets at no cost and if they made a msitake, end users should really uphold what the author wants, and work within the newer/updated license framework if its possible. If for any reason this makes it difficult because of the project logistics then perhaps some negotiation/leeway can be spoken about and communicated of the users intentions.

If you have treid to reach out and done what you can to ressolve your concerns and no one gets back to you, then id go with what you have, if its CC0, then so be it. You just need to make sure that the uploader is the geniune owner of uploaded works.

This is my opinion, not real legal advice :) 

I hope this works out for you, but most of the issues you're working through have little to do with OGA, so there is almost nothing we can advise you on more than what has already been said.

"...I got in touch about it and will come back here with an answer so that other users don't have to go through this kind of situation..."

Thanks. But other users already don't have to go through this situation for the assets in question. I mean, you don't have to go through this situation either, but it sounds like your publisher(?) is convinced otherwise.

So there is a little misunderstanding here in that the licensee who posted the material is allowed to share their materials under any license they want, wherever they want and they do not have to use the same license all the time. 

If they want to post material here under CC0 they can.

If they want to post materials on their own site with stricter licenses they can.

If they want to sell the material to an individual with a custom license they can.

That is all fine. You just have to make sure that YOU, as the user, are using it under the license that you access it under.   The creater holds the copyright to the material and they can license it however they want to whomever they want and the license can be different.

I'm curious,...

If you can sell a game with those assets inside, does that count as reselling?

And if you sell those assets all by themselves, shouldn't that be the only reason to get called out for infringement?

"does that count as reselling?"

Losely interpreted, yes it could, which is why none of the licenses on OGA allow for extra stipulations like "no reselling."

sell those assets all by themselves, shouldn't that be the only reason to get called out for infringement?

I understand what you're getting at, and its a good point, but... Nope, that isn't the only reason, because it isn't a reason at all. Because even selling the assets by themselves isn't infringement of the license. If the license says "no reselling" then it isn't a CC0 license, is it? It's like saying "Well, it's totally free, as long as you pay me $50.00" Well, then it isn't free, is it?

The assets here on OGA are openly licensed. If you get them from here, they can be resold, packaged with a game or otherwise. The terms of use on assets you get from somewhere else are irrelevant here. It doesn't matter if they appear to be the exact same assets as differently licensed assets elsewhere. If there are weird terms attached to assets on some other site, don't get them from that other site. Get them from here instead.