Do not fit, and Not as Described are two very different things on eBay, especially if you are after a refund.
Not as Described..... Did the listing for example state one size and on arrival, another size is written on the clothing label? Then, here you can open a case for... ' Does not Match Description or Photos '. The seller Has to refund within 3 days or provide a return label. If not, you escalate on day 4, eBay step in, and force the refund.
If it's just a case they do not fit, if the seller has a No Returns on their listing ( do they? ), then they can refuse the return.
@zeus64