I want to preface this post by clarifying that it's not an atempt to defend any of the seller's actions that have made the transaction more difficult than necessary, I just want to post information so that buyers in general might have a better understanding of some of the actions a seller can take in certain circumstances, with the disclaimer that it is general info and may or may not apply to this (or every) seller.
Marking as posted - many sellers use this function as part of their order processing, to update the progression of an order but not necessarily exclusively to indicate to the buyer that the shipping service has the package. If I have packaged an order, I mark as shipped, no matter what, because at that point I have used packaging materials, printed invoices and paid for postage, so changes to the order, or cancellations, are more difficult to deal with and incur more unrecoverable costs if changes or cancellations are made (also, if it's not marked as shipped until I get back from the post office - typically hours after I leave - and someone has asked to cancel in the interim, an answer of "too late, already shipped", then marking as shipped is likely to be interpreted as reluctance, not impossibility). Other sellers will have booked courier or Aus Post collection, so to them the package may be considered as good as shipped.
Sellers don't get order progress / status updates that we can apply to orders that are in various stages, so marking as shipped is the only way to update an order that's at a point where a seller wants to clear it from their "awaiting shipment" page and view only orders that still require the initial stages of order processing.
Opening requests - I understand that in this case, it got to the point where it seemed the only option, but eBay requests are not an exchange / replacement request, they are designed to be refund requests and don't cater for any other option without risk to the seller. Some sellers will still accomodate those things once a request is open, but I won't, nor will many other sellers who would be very happy to facilitate those things under any other circumstance. I only mention this to help explain why a seller (good or bad, objectively or subjectively) will only offer a refund when a request has been opened - it's because they can only offer a refund, unless they are prepared to risk their selling account.
Did the seller accept returns? If the matching linen hasn't been opened / washed, you could request to return those under the seller's return policy.
