We can't really help you with a value without seeing photos of the Zenith pool table.
They aren't worth much, in general, but it all depends on it's condition (which really can't be assessed without a photo). Unless the condition is immaculate (e.g. like new), it generally won't bring you much beyond what any other 1960s/1970s style coin op pool table.
In general, the Zenith brand isn't really well known, and there generally isn't any specific demand for this company's pool tables.
Additionally, the market is flooded with 1970s-style pool tables, so it's a buyers market. In most major cities in America, you can find folks giving away used pool tables for free. That makes it hard for sellers to get a good price, but it all depends on where you are and what else is available in the local area (e.g. within a few hours of driving distance).
So generally, you'll get less than $500.
One thing you can do to roughly price it is to look at local classifieds for a similar-style coin-op pool table (of the same playing size). Price it in the neighborhood of what others are selling for.
For ease of maintenance, coin-ops usually use a one-piece slate and can be moved after disassembling the feet only. If you can't get your head & flashlight inside the table and see the underside of the slate, your next option is to try to find seams on top that would be present between thirds of the slate.
Your stairs complicate the move, however, and the good news is that that slate can be removed relatively easily and go up separately. Two people can carry a slate given the right carrying apparatus, but a trip up stairs must be planned in detail. I'd guess that the slate is three-fourths the weight of the entire table. Check YouTube for videos, possibly "Moving a coin-op pool table", including: