Under Sale of Goods Act 1979, all items sold must be "as described", this includes second hand goods. A no refunds policy has no bearing against your legal rights under this act. Even if the seller made a genuine mistake, they are in breech of their sales contract with you.
It doesn't matter if the label says 34R, if you can legitimately argue that it doesn't measure the standard 34R sizes for a specific brand. The label may be incorrect or the suit altered. The purchase was made on an item described as 34R
On the other side there is a label saying "slimfit".
The arms are so short, I'm wondering if DS is actually a long, so it may well not have fitted anyway...I always knew I was taking a chance buying a suit that hadn't been tried on, and really don't want to be an arse about it.
It's not being arsey if the listing was factually incorrect! Why don't you message the seller to say it's has arrived but unfortunately doesn't fit because it's an S not an R and put the ball in their court. I suspect the resale on a S will be much less than an R, so don't assume you can resell to recoup your money
Writing 'no returns' in a listing is meaningless as eBay will just force a seller to accept a return if the buyer requests one.
I would simply email the seller appologise and point out the mistake and ask to return, if they refuse open a not as described case. Post it back recorded/signed for, give eBay the tracking number and as soon as it has been signed for eBay will issue the refund.