The seller should be responsible for moving. either for free or with a small fee.
I don't really get the purpose of making the Cross Server sub if the buyer has to move anyway, as I don't see it provides any added value.
It would be no difference for buyers who follow both Silver and Gold before the Cross sub was made, and make the transfer themselves.
On the other hand, the Cross sub as it is now also diluted the value of individual subs. Why should I make a thread in my own server when I could post it in the Cross sub and gain more exposure (in the future) because the buyers will have to move to me anyway?
Thus, in order to make the Cross Server sub has its own purpose, the seller should be responsible to move, in my opinion.