It isn't etiquette, it is eBay policy.
Second hand clothes are meant to be cleaned thoroughly before being sold on eBay.
How long ago did you contact the seller? Give them a little time to respond, as it's the weekend, and they might be busy off-eBay.
You can contact the seller once more, and mention the issue, very politely :), and see what they suggest. Hopefully, they will be prepared to contribute towards or cover the cost of cleaning it.
Be careful how you word it, and do not mention anything like "before I leave feedback" or "I'm thinking of negative feedback", as that could be construed as feedback extortion (against eBay policy too).
Personally, I would obtain a quote for a dryclean from a cleaner near you that you trust, as it is silk, so you know how much it would be.
Did you pay with Paypal? If you did, bear in mind you have 45 days from payment to open up a claim for item not as described, but this has to be significantly so.
If the seller will not acknowledge there's an issue, you can try a Paypal claim; but I am assuming you want to keep the dress, and would not be looking for a full refund (you'd need to send it back tracked for that from Paypal). Paypal will not offer a partial refund, you would need to have the seller be amenable to that, if you want it cleaned at their cost.
The seller does not have to give you a part refund for cleaning.
If you think it's warranted, and you are not able to work things out with the seller, you can also report the issue to eBay, as it should not have been sold uncleaned.
Do not clean the dress yourself if you think you might open a Paypal claim, as this would alter what you received, and void your Paypal buyer protection.
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Buttercup: You mock my pain! Man in Black: Life is pain, Highness. Anyone who says differently is selling something.