Would you buy, say, a saltwater fish tank on Craigslist? Would you trust that everything is in working order?

Well, for $150 what have you got to lose? I would go for it.

I’d fill the tank to be sure it is not leaking, otherwise, sounds like a great deal. People sometimes just want to get rid of stuff. I am planning a yard sale soon and am selling a $150 drum for $25 because it is just taking up space and I haven’t played with it in several years.
Nothing wrong with it, just no longer wanted in my drum collection.

I’d ask outright if the tank has any leaks and if it does leak once you fill it, can you return it to him, say in a week or two. I agree with @Coloma, most people just want to get rid of their stuff even if it works fine. That he’s giving a bunch of extra stuff would lead me to believe it’s all working fine.

I am going to go ahead with it. I’m pretty excited. :)

It is quite an expensive hobby so perhaps they do just want to get rid of the tank and accessories. I would do as has been suggested, ask if it leaks, perhaps ask if they can fill it with water before you arrive so you can check it over with water in it if you are really worried. At $150, sounds like a good deal. Perhaps you could do a bit of research into how much it normally costs to get a tank repaired if you are concerned.

You will have to take a photograph of your beautiful saltwater fish. I have never been game to go down that road because the fish are SO exy and so sensitive.

I contacted the fella and haven’t heard anything back. Perhaps, he sold it, but didn’t take down the ad. Bummer.