Although I've had a 1980 CB750, I never rode it. I went through a stage when I bought 4 of the early DOHC 750's with the intent of fixing and selling them, but ended up making some money without doing anything, so I unloaded them. I did have an '80 CB900C, and I think there's plenty of similarity with the 750, so I think I might have enough experience to at least get the conversation going.
My first thought is that if the bike actually sparked before having the work done to it, the problem is likely with the rework... problems popping up after something is changed is usually related to what was changed. However, the rebuild process touches a lot of areas. If the coils were new and part of the rebuild, even they could be suspect, or maybe just not wired correctly.
So, the things I would consider that in your case might cause a non-sparking issue include a wire not connected, wire(s) not connected correctly, spark plug wires from the coils not connected correctly, bad spark plugs, bad CDI units, pulsars, or any combination of these. I do believe that it could easily be something simple, yet maybe hard to find, again assuming that things were electrically fine before the rebuild.
As to be being worth $600... I would think not. First time bike buyer after a non-rimming bike sounds pretty risky to me. However, you may be excellent with wrenches and just asking for suggestions, all the while already having an idea where to start. Or maybe you have a particular reason for want this particular bike. My point... it's your call.
For example, I mentioned that I had a CB900C. I bought it as a non-runner for $250. I got it running for no cost, but I'm sorry to tell you that I really don't remember what I had to do. It took quite a bit of time to get things sorted out, but mostly because I didn't know a thing about the DOHC bikes. I remember rebuilding the carbs and re-cleaning them a total of maybe 6 times before I got it running well. I had to re-shim the valves to get them set correctly... I don't care at all for the way valves are adjusted. In a nutshell, I don't care for the DOHC bikes because of carbs and valves design.
Don't take my opinion for anything more than it's worth... it's mine only, and I'm sure there are others who have just the opposite opinion.
My first suggestion is that if you're a first time bike buyer, unless you really do want something to work on and learn what makes it tick, get one that runs. Be sure there are no strange noises, it shifts well, stops well, and doesn't appear to have more things needed than you want to tackle. I'd tell you that there are LOTS of nice bikes available for a decent price, and you should be able to come up with something nice to suit your need.
