Try and see if the selling agent will go for you including in an agent and see how it goes.
Working with the listing agent means you are working in a disclosed dual agency situation.
In NJ we work as disclosed dual agents frequently. To us, it's not a big deal. It seems to be a bigger deal to agents in other states. My understanding is, that NY agents, also can worj as dual agents.
Moving forward, if you don't buy this particular coop, then by all means find a buyer's agent to work with. You still may wind up in a dual agency situation if that agent happens to list a property you want to see.
Unless you are uncomfortable with this seller's agent, then I'd say go ahead, and make your offer. The listing agent might have a better understanding of the coop board, too, so it might be to your advantage in the longrun.
It would be wise on your part to find an agent who will work on your behalf.
Remember, although you can work with the sellers agent (and if they work for you they are obligated to look after your interests as well), they were hired by the seller.
If you can't find a Realtor you are comfortable working with you can contact me through my profile I can recommend a Realtor who would be able to assist you.
Hope this was helpful.
Ideally, you'd start out with a buyer's agent, but to bring one in now is probably a wash. Unless they have some experience with that particular co-op, in which case they may be quite useful and helpful.