anyone here that can help me through this?

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I would like to make an offer on a property.
I have the address and the listing co and their phone number. I can't find it listed on mls or a co site though 2 nearby ones for sale are listed on the co site. (I drove past a bit ago to be sure it was still listed.)

I have only an idea of the list price, guessing 180 and up. Zestimate is about 165, I'd be offering less.

What steps do I take? Should I get a buyers agent or work with the listing agent/office? No agent is named, just a phone number.
edited to add, its had the co sign on it for over a month.