" about $30,000 more than the property tax value "

Tax valuation rarely matches market value.

"ll the other lots in the neighborhood sold for between $40-50K"

Recently?
That would be a better indicator of market value.

How can I let the sellers know that I would definitely buy it for $50K, but not their $70K?

You could tell them verbally, you could write a letter and send to them, or you could send them a contract offering that amount.

Send them a written offer and a few comps.

What NC said. Put it on paper with the evidence that your offer is logical. When I'm selling stuff and people ask me what my bottom dollar is I politely say "I'm asking X." There's no point in negotiating with yourself by dropping your price before they've offered something. So that might be what they're thinking.

Though on a house or land, my response is "make me an offer and I'll consider it." If they ask "is X included" I say "you can put anything in an offer."

So put something in writing in front of them. That shows you're serious and may budge them off their price point.