Yea, that's just wrong to switch machines on you like that without giving a good reason and then ASKING you if it's OK first. Perhaps something was wrong with yours that couldn't be fixed and so he wanted to use it for parts, but that's still in no way a good reason to do what he did without getting your OK first. More than likely, though, it sounds like he sent you a beat up machine so that he could keep or sell a better machine. What other reason would he have for doing what he did? I'm sure if yours had a problem he would have called first to let you know, but by not even contacting you about what he was going to do that sounds like something shifty to me.
You're biggest weapon to get him to make things right is to make sure you post this info on all the main detecting websites so that he can no longer get customers with his bad reputation. Keep hounding him until he either buys you a new machine, sends you back your original, or at least sends you a machine that is in as good of condition as it was. I wouldn't let that sort of thing slide myself either so I hope you are very vocal about what he did to you.
I"ve got a small story for you...Several years back I sold a very new Explorer off. It was my third and it saw very little use because I was quickly reminded about why I didn't like the first two and the performance they were getting in my soil. Anyway, I put it up for sale on one of the forums and mentioned it had very little use and was like brand new. I can't remember but I might have put some pics of it up too. Anyway, a guy PM'd me and offered to buy it for a slightly lessor price, which was already I think like $250 off what I paid new. I agreed but told him I wanted a money order for it. He agreed and I shipped the item.
Well, he didn't send a money order but rather a check. Before I knew it he emailed me and said he was stopping payment on the check because the machine wasn't in the condition I said it was. He supported this by showing me some dirt under the coil cover that I never bothered to clean. Hey, I smell a rat here. I never said the machine wasn't used, I said it was like brand new with very little hours on it. I even sent him the original receipt to prove it. He responded by saying that the receipt looked forged. Ended up he called the local dealer I bought it from and chewed him out, and as a result that dealer chewed me out. For what? That WAS the receipt he gave me, but the dealer didn't like how the guy was implying that something was fishy and he might call Minelab on him. Either way, as a result the local dealer I had been friends with and I parted ways. All because some jerk with paranoia thought something was wrong with the receipt. Really I knew he didn't, he was just looking for ways to not have to pay for the machine or to at least talk me down to a lower price than what he had already agreed to in the first place.
Then he started complaining that I didn't send him the Minelab hat that came with the machine, which I didn't and had forgot about and told him I'd mail it right away. Still no dice, so I told him that I was not going to bargain on a brand new machine that I already was selling well below what it cost new and that he should mail it back to me. He refused to do that as well and said he was going to turn it over to the local police because why should he have to pay to mail something back to me that he didn't want to buy at the agreed price.
That's when I decided to play hard ball. I called his local sheriffs office and explained the situation to him. The officer told me that there were mail fraud laws that would protect me in such an instance and that he would have me fill out some paper work to charge the guy with check fraud. I Emailed the guy and said what I was intending to do if he did not return the machine to me. Suddenly he was willing to pay the initial asking price and settle things. As a jesture of good measure I told him to knock $25 more off the price after that. You see, I will not be threatened or blackmaled into doing something but once he changed his tune I was willing to give him $25 off, which I shouldn't have done. I mean, that's pretty expensive just to clean between a coil and cover.
Later I did some checking and found that this guy was a seller. He wasn't buying that machine to use it. He was buying it to re-sale and as a result he was trying to get every last penny out of me so he could turn a bigger profit. I have since made sure I do a few IMPORTANT things when buying or selling metal detectors. First, clean every last bit of it or make darn sure you list every little problem (even dirt under the coil cover). Don't give them any excuse to say they didn't get what they paid for. List every little detail. Second, it's easy enough to click on that user's name and sift through all his old messages. See how good of a reputation he has. If he doesn't have a history then that's a red flag. Third, I try to avoid any guys who are just buyers and sellers. They way I see it they are more apt to be the ones who are going to try to drive a hard deal, work you on the price, and so on. Not all these guys are this way, but you MUST check their history out first. Had I done that with this guy first I might not have done business with him, because he didn't have the best of reputations and his asking price for used items he was selling was almost the new price. That should have been a clue as to him looking to work anybody over the coals that he could.