If there is no reply at all but you can see the order on bricklink and it hasnt
changed status to say shipped then double check the sellers terms page in case
they have mentioned any issues. After that use the report a problem and start
a non shipping seller. That may get a response but then if that still doesnt
then you need to file a dispute in paypal as well to get money back.
In Problem Order, land48 writes:
Since the 20th of June, I made a purchase, having paid by Paypal. Since that
day the seller has said nothing. The purchase is as paid, but for 10 days nothing
I already sent two messages and he didn't even see them.