So I looked up "10 bolt 7.5" in ebaymotors just to see what pops up, i want a set of gears but dont like the cost of the ones in jegs and summit. They have a lot of them for under 100 bucks, some with a buy it now @50. Are used gears a bad idea???

yea used is bad, from experence, you dont know how they were set up and what the specs where on it when it was removed

go new with summit or jegs, or maybe new on ebay

but keep in mind sometimes you need to change the carrier or get a set of gears that are made for the carrier you got

if you got 3.73 now than any gear from a 3.08 and up is for that carrier

Used gears are harder to set up because the contact pattern will not be easy to read. Getting the pinion depth set correctly will be harder. It can be done, but you need to have some experience with used gears. I also don't recommend the cheap Summit and Jegs gears. If you want gears that will run quiet and last a long time then get good quality gears and set them up right.

The majority of the "cheap" gears on Ebay are used factory gears. Although they'll usually work just fine, they're not a better aftermarket gear. For the 10 bolt, the ring and pinion teeth are small and even a good aftermarket gear is not indestructible. The used factory gears are usually cheap because they're in a ratio that very few people want.

My 9" has Ebay gears in in. I found a set of US Gear 4.86 lightened pro gears that were polished and debured. Even though they were used, they're still fine to use for me. A huge saving over a set of new similar gears.

Big Gear Head basically summed it up correctly. Personally, I'd never buy any of the new gear sets on ebay that are priced around 100 bucks or less. I guarantee you all of those are Taiwanese or Chinese junk. You'll notice almost all of those auctions never even give you a brand name at all. Gears are one area where it's not worth the risk to scrimp on the cost. As stated above, used factory gears are sometimes ok, provided someone with lots of setup experience is doing the install. The last gear install I did, I used a brand new set of Motive brand gears, which are good quality, and they set up beautifully. They're perfectly quiet and plenty strong. I used Richmonds in a rear I had set up almost 20 years ago, and they're still running perfect too. You get what you pay for. Btw, I bought the brand new Motives off ebay for substantially less than what Summit or Jegs wanted for them.