The buyer should still be able to collect it.
When you wrote the address, did you include the eCP code as part of the address?
As long as you did that, the shop will be able to accept it and match it to your buyer.
You may need to contact Ebay Customer Services to ask them to change the tracking from the Ebay label to your RM 48 tracking number.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)