I make a point of talking with potential buyers and sellers on the phone prior to closing a deal. While there are still no guarantees, once you've spoken with someone a couple of times, you get some sense of who they are, which can add to the comfort zone for both parties. I've sold and bought equipment both COD and cash up-front, and have yet to get stung either way. As others have mentioned, the only hassles I've had were with UPS' timeliness in returning COD payments to me. I much prefer to use FedEx, although they are apparently not yet set up for COD service on their ground shipping (formerly RPS).
A piece of free advice (and worth every penney): If you ship bulky, heavy items, such as speakers, to a pre-paying customer (and especially if you're shipping overseas), have your customer arrange pick-up and shipping from his end via his favored shipper - that relieves you, as seller, from any shipping problems that might occur in transit (i.e. no post-sale headaches).
A piece of free advice (and worth every penney): If you ship bulky, heavy items, such as speakers, to a pre-paying customer (and especially if you're shipping overseas), have your customer arrange pick-up and shipping from his end via his favored shipper - that relieves you, as seller, from any shipping problems that might occur in transit (i.e. no post-sale headaches).