I would like to get another 8.75 center section,( a used one) sure grip 4.10 gears. What do I need to look for/ to make sure I'm not getting ripped off?

Buy a new ramanufactured one. Everything with a "good" used unit COULD check out, then you have the chance of putting them in only to hear them howl or something. I bought a good used set of 3.91's years ago that looked good, had no pinion play, good looking ring gear with no pitting in the teeth, and they howled away endlessly. Used ones are so expensive now, you'd most certainly be better to buy new for only a few hundred more. Check out DR. Diff. He's a member here too

Unless there's some obvious damage you can't visually tell if a gearset will be quiet/noisy.

ttt

I like picking up 742 cases with the dana powr-lok sure-grip. I think they are overall the best factory package. You can't really tell with used gears if they will make noise...I haven't had issues so far.

AVOID anything with a CONE TYPE SURE GRIP like a case of the CLAP ...

i wld go with new, my story is, i bought a used 489 3.55 sure grip unit for 250.00, look great however i took to the rear end shop, the inside gear was gone, i spent over 600.00 getting it rebuilt with all new , now i have 850.00 in used , when you can get new for about 250.00 more,

if I had to do it over again I'd buy new...I went used, got a 3.55/sure grip 489 case. works good, but the gears have always been noisier than I really want, and now after about 20k miles years later, the sure grip is getting very weak.

Just to put things in perspective, I just sold a very good used 742 case 3rd member. I had it recently setup with new 4.10's, bearings and a rebuilt disk sure grip and a large yoke. I got $600 for it. I thought that was a good price for the buyer.