Welcome to the board.
An accident car does not necessarily mean it's bad, however it also can mean many other things. If you know for fact what happened to the car, or know the salesman personally then it may be ok to trust this guy. If you do not know anything about it, then you could definitely send it to BMW to check out for you, however because he states there was no frame damage doesn't automatically make the car sound. There are many other things that can be affected long tems such as rust, more prone to stone chips, leaking window seals, door seals, etc..If i was in your shoes, i wouldn't rush into it. Just keep looking! Patience will bring you something nicer.