Diablo and Diablo uMax completely different animals, make sure you know what you're bidding on. The original Diablo was kinda sorta okay back in the stone age, not worth $25 now. The Diablo uMax was introduced in early 1996, and is still regarded as a good gold machine. They're particularly good in noisy ground. Unless your hearing is really sharp, you'll need headphones, it's not very good on the speaker.
For reasons I never understood, only a few Diablo uMaxes were made. The ones that did get into people's hands almost never go up for sale used, those who have 'em won't let go of 'em. When they do show up on the used market, they command a premium price.
The Diablo uMax is strictly a gold machine, and without having a discrimination mode most folks would rate it as useless for relic hunting.
******* a bit of a reminisce *******
About 2000, a used Diablo uMax came up for sale for (as I recall) about $350 from a reputable dealer. I was not feeling rich at the time and didn't spring for it. Took me about a week to realize my mistake and began kicking myself in the butt over it..................until about 2008, when Jim Straight heard how I was still kicking myself in the butt, he was getting too worn out to do much gold beeping any more, and gifted me with his very beat up but still perfectly functional personal unit. Which I can assure you I still have and it is NOT for sale!