I can almost echo your experience and it was a major turnoff for me too. It was the first thrifted thing I have ever bought, and it is very likely to be the last. I should have realized how dicey this would be for me since I have allergic contact dermatitis to an ingredient that is so ubiquitous in laundry detergent it's seriously hard to find one without it. It's also in Febreeze. Outside of that, I just can't stand fragranced things.
The item was not expensive and was a top I really wanted, a different print of one I already owned, so I decided to try and get the smell out. I should have taken your route and returned it because that was a lot more difficult than I thought it would be. I did multiple washes, soaks in baking soda, soaks in vinegar. That thing was soaking in something for at least 5 days and was washed by itself probably 8 times.
Guess what...still haven't worn it. The smell finally stopped but by then I was afraid of it and turned off by it. I did contact the seller and she said she used fragrance free laundry detergent. Yeah, right. If I believe her, maybe it was Febreeze...didn't think to ask about that. Or maybe she wears such strong perfume it's in all of her clothes and she no longer notices.
Anyway, you were smart to return. I'm done with thrifting I think. I admire people who can do it and have success but obviously I have too many issues to be one of them!
I also have a complaint about Poshmark specifically, the way it will only accept positive reviews. Sellers can look so good but that is only because the site hides the negative ones. Seriously dishonest and misleading. Done with it!