@dimitrisarv
Buyers from all servers should comply with that and will have the responsibility to transfer server in case they are from different servers, but still offered and became the rightful winner, unless:
Seller announced the Pokemon sale/auction on one server, but transferred to a different server before they complete their part of the deal, regardless the reason.
There is an explicit written agreement between buyer and seller that the seller will transfer to honour the trade.
The original post clearly states, that responsibility of transferring lies with the seller.
Since none of the 3 conditions apply in this case and as I've mentioned in the start, the buyer does have to transfer if they're from gold upon winning.
Get this right that the responsiblity of collecting the poke depends on the buyer for all cross trades. Its written clear as the blue sky in the rules.