How much room to negotiate with a third party agent?

Hey guys,
My longtime salesperson from my local Merc dealership has joined a Leasing company. My lease is coming up later this year (Thanks for all your help on my other thread!) and he has already pinged me to contact him when I’m ready to buy. My question is: “How much can I negotiate?” I’ve never dealt with an agent before as opposed to a dealership so I’m in completely foreign waters. If I decided to go with one of the great posters on this forum (I’m located in NJ, so it seems like there are a few agents I could potentially utilize here) are those deals they post even further negotiable? Hoping you guys can further remedy my ignorance.

An Example of what his company is offering on their website:
2019 BMW 530XL, 10k per year, 36 Mo’s, 0 Down TTF DAS 603 Per Month (MF/Residual, MSRP, ETC not listed)

An Example From this forum:

The agent is simply working on your behalf. You can negotiate all you want…it’s up to the dealership to take the offer or not. All the agent is doing is playing middleman for you.

That makes sense. I’m not super keen about adding a middle man into the equation. Logically speaking it seems like it would inflate the price as one more person now needs to get paid. There aren’t usually exclusivity contracts or the like that need to be signed, right? I could always go to my usual 10 dealerships, get the best offer and compare it against what he is offering, right?

I have no idea what type of deal you have set up with your agent, if you have one at all. That’s completely up to him on how he handles his business. He may ask for a retainer to begin working on your behalf. There’s nothing stopping you from working on your own as well though, outside of possibly losing a retainer fee should your agent charge one.

Brokers/Agents do add an additional cost, however, there are times that they can get a better deal than you could based on the relationships they built up with their contacts. They are also beneficial for someone who doesn’t have a lot of time to negotiate, and the price they charge could be well worth it to you to save you some time.

Thanks for the response. I haven’t made any deal with him yet. I’m trying to understand the way that these relationships work before entering into a relationship with him. I like to always come to the table with as much knowledge as I can to best represent myself. But Based on your response I think I get the gist of how to tailor my expectations. I truly appreciate you taking the time.