I have bought 3 used MD's and a few used coils on E-Bay in good faith and the best I can say is ask a lot of questions, especially if there are not many pictures. If you are buying used, you have to keep used, not new, in mind all the time.
The first one (BH 505) was advertised as new and was not but I did get it for less than half of new and it works great though it had a couple of minor cracks in plastic. Considering my time, travel, and postage to send it back, I just warned the seller and kept it.
The second one was a Garrett Master Hunter ADS VLF-TR Groundhog. This unit was pretty well described and arrived as such and works great. In air, I can get it to hit clearly on a 2 grain gold nugget with the 4.5 inch coil. Still have not had time to bury my test piece an inch or so deep to see what it will do in mineralized soil here.
The 3rd one is a White's V/SAT Gold Master. In some part, I feel I should have asked more questions as it was a little more in the rough than the two pictures disclosed. For the price though, I do not figure I was hurt too bad as I've seen GM II's (about 20 years old) go as high recently as $490 + shipping and my V/SAT that is 5 years newer I got for half that. Even saw a V/SAT yesterday go for $292 and the shipping was about another $78. Shipping for my V/SAT was about $10 or so. I did have to clean it, remove the remnants of some kind of paper stickers or label's that were not original, and the cuff bolt was missing the original plastic threaded knob and had a simple nut in it's place that was not shown in the pictures. Ran to the hardware store and bought a new knob. The seller also said he was throwing in a set of headphones though arrived broken but I was not interested in the headphones as I have plenty of my own. However, not mentioned in the Ad was the coil came with a cover. Another plus and not mentioned in the Ad was it came with 8 AA rechargeable batteries that did recharge. Funny part is thinking back, I think I saw this very machine on E-bay over a year or more ago. All the functions work, in short order on a mountainside in a wash down to bedrock I was finding small bullet fragments in mineralized ground so am happy with it's performance. Have not had time yet to use my test nugget but have no doubt it will find it. This thing is so easy to use, I can't believe it but then I'm used to manual ground balancing.
One coil I bought used was for my Groundhog. It was advertised as a 14" Groundhog coil. It also had the right size connector with the right number of pins. When I got it, it was clear it was not a groundhog coil as it would not mount correctly to the shaft without spacers added. It also did not work in VLF mode but did in TR. I researched the best I could from old pictures and decided it was a Deepseeker Coil that will not work on a Groundhog unit. I contacted the seller and he immediately got defensive and said I was wrong and that it was a Groundhog coil. For the cost, I just kept it and if someone someday wants it, they can have it.