Is it fair to assume that an English speaking vendor at this market will be
hard to find ? *I should take appropriately worded cue cards ??
That's hard to say, but there are a lot of people in Beijing studying
English now, just because of the Olympics. So you can probably bet,
that at least some people there will speak a little English. Still,
it's best if you bring cue cards to avoid confusion.

Secondly ... I understand that bargaining in Beijing is the norm in the
market type stalls, but is it still appropriate to bargain at the tea
market, which I assume the locals also shop at ? *I wish to avoid offending
anyone.

If the product doesn't have a sticker price on it, assume that you can
bargain down the price a little. Make excuses like these:
"It's ugly, I don't really like it. Give me a cheaper price."
"I'm buying a lot of them. Give me a cheaper price."
"I will tell all of my overseas friends about your store. Give me a
cheaper price."
"At the other store they sell it for (x) yuan, your price is too
expensive."
"This one has some defects/chips/cracks. I want a cheaper price."
"You don't have the box? I want a cheaper price."
"Your boss said he would give me a cheaper price."
"My wife won't let me pay that price. But if you give me a cheaper
price, maybe I can afford it."

You get the idea. Be creative. Of course, they will also make excuses
why you should pay that particular price. So be prepared. It also
might be a good idea to check prices in a few places, just to be sure
the price they are quoting you is a fair price, and not an escalated
one.