I'll usually clean the bearings but almost never replace em. It's good to get in old wheels and clean out the stuck old grease, clean em up and repack but unless theyre rust,y pitted or grooved on the cones I dont replace anything, just clean and new grease. On bicycles i'm a little more willing to replace but that's usually cuz i'd have bought caged bearings in lots, and it's easiest to grease up a new one and throw it in. even then it's the cages that're usually the worst problem, loose bearings i'd still clean and repack with new grease whenever possible.
If it's sealed bearings though, I'll replace those again just cuz theyre cheap and it's easier. But theyre also alot less likely to have issues and dont need readjusting the way cup and cone loose bearings do
And on quick flips, it's often just not worth the extra time. you're not gonna get anything but a little peace of mind to spend the time and money on it, and whoever snags it isn't gonna notice. IIn those cases it's the buyer's prerogative- if they're planning on building a keeper, you do the details