I would consider it rude... but...

maybe they didn't receive or missed your e-mail?
or maybe their reply to your e-mail bounced back?

the last thing has happened to me several times
(both offering and receiving offers), and when it
happens there's nothing i can do..... Well, if i'm
the one receiveing the offer i could post a
message on my gallery saying something like "Hey,
your e-mail is bouncing back", but of course it's
not something everybody would like to do....

It also happens the other way.....

1- You receive an offer on a cel or asking for a
price reference (sometimes asking to send bigger
scans etc..).
2- You reply politely with your price, or a
counteroffer (or sending the bigger scan etc...)
3- You never hear back from that person. Maybe a
short "Sorry, that was more expensive than i
expected" or similar would let the gallery owner
know that they are not interested on the cel anymore.

As for the answer of your question, if the
buyer/seller "don't want" to reply i consider it
rude, but many times i have no way to know if "he
doesn't want" or "he can't".