An interesting advice I've gotten from a tea merchant about places like
Ten Ren is this -- buy their cheapest tea.

It might sound weird, but if you buy their cheapest tea, it's likely to
be fresh because people buy it all the time. if you buy their
expensive stuff, it's likely stuff that's been sitting in the same
canister for ages because nobody buys them.

And thinking about it, it makes a lot of sense.