Doesn't actually matter if they really think Coyle ratted them out. They're already talking about getting out of the business, at least for a while, and he's the link to the gun buys. So if he gets hit, that can only help them. Sort of goes along with the whole idea of Coyle's tragic uselessness exactly at the point where being useful is the only thing that could keep him out of prison and alive. Brutal, but predictable and very real-life.

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My take: In the car on the way there, Scalise and company confirm that they know Coyle could not possibly rat them out because a) Coyle doesn't know the details of their caper and b) "he's a stand-up guy." (And don't forget that Foley and Waters had talked earlier about getting a warrant for bugging Dillon and the bar.)
In prison, Scalise and company ratted out Coyle to the Man (the police)not to the Man (the mafia)no doubt to lessen their own sentences.
Not sure that Jackie Brown (the gun runner) was indicating Coyle at the end when he swore that he would "kill him" and "handle things" in his own way; Jackie could as easily be talking about his own supplier, who seemed pretty untrustworthy in his own right.

I think that it's a good twist with Dillon being the rat, and then making the hit on the guy who they think is the rat.

Why do you like how they changed it?
"Two more swords and I'll be Queen of the Monkey People." Roseanne

Yes, that's how it is in the book. She tells a State cop about it.
I actually like how they changed it in the movie, though.

I read somewhere that the beautiful girlfriend of the robber who bought the guns from Eddie Coyle talked ot the authorities.
"Two more swords and I'll be Queen of the Monkey People." Roseanne

I need to watch this again. Yeah, they figured that Coyle is the only one who knew about what they were doing, and had a good reason to trade them in. They knew that they didn't tell him where they were going next, but assumed that he figured it out somehow.
The movie (different than the book) has the nice irony of Dillon ratting them out, and then killing Coyle, the supposed rat. It's also good for Dillon, since he could stand to lose if Coyle gave him up in NH, on the stolen booze.

On the way to the house where they are trapped, the bank robbers discuss the fact that Dillon has warned them Coyle is going to sell them out. So when they get trapped, they probably think Bingoits Coyle. Nevertheless, as Scalise pointed out in the car, Coyle doesnt know where weve been until weve been there. So Im surprised Scalises lawyer was convinced enough to tell the Man with any certainty it was Coyle.
Exactly how the cops knew where to trap the robbers is not explained in the movie. Its definitely Dillon who fingered them, but it seems unlikely that they would have told Dillon their precise target. I figure either Dillon overheard something, or else the cops did tail themperhaps, for example, by sticking a homing device on Scalises car.
As to why they kept buying guns for each jobaccording to the novel, they did not want to be caught with weapons between jobs. Scalise and Artie were known felonseven Foleys boss has early suspicions that theyre behind the heistsso theyre just playing it very safe between jobs. They dont acquire weapons until just before they make their move, then they immediately get rid of them. In the book, Scalise is even having his girlfriend dispose of the loot by making small deposits in various banks under assumed names. They want to look totally innocent if theyre picked up on suspicion.