Well, if it was that way when the fellow you bought it from sold it to you and he didn't tell you, then that's cause for being more than a little cross. But that's a different axe to grind.
Sonically, it really shouldn't make much difference at all, though. Largely cosmetic. Not sure what B&W tweeters look like, but it should be relatively easy to pull out the dent if it really is bothering you (as it would probably bother me). If you're careful about it, you should be able to gently tap the corner of a piece of regular scotch tape onto the dent and then pull it out. May take a go or two, but it typically works very well. It may not look perfect when you're done (you'll likely have some residual creases or imperfections) but it should be much better.
Might be worth contacting the seller first and explaining that the dented tweeter (or undisclosed damage to gear generally) really wasn't part of the deal and that, while you're eager to work it out and even try to fix it yourself, you are dissatisfied and it needs to be worked out one way or the other. The sooner he knows the better and the easier it will be to make everyone happy in the long run, which is, after all, the way to go. Best of luck.
[Also, while you're at it, do a search for "dented tweeter" right here and you'll find a long thread discussing just this with even more (and I dare say better) info and advice.]
