The bearings not spinning smoothly will affect the trackball performance more than worn rollers.
Check the bearings, to see if they are spinning freely, before dissassembly.
If they DON'T spin freely, replace the rollers and bearings all at once.
There's no sense doing half the job.
If they DO spin freely, I'd take the rollers off, put the ends lightly into a drill chuck, and then polish them with some 150-220 grit sandpaper, to smooth out the dip in them a bit.
NOTE: the rollers will get REALLY hot while doing this, so don't touch them.
I usually rip the sandpaper into strips, and hold the ends of the strips, using the middle area to do the polishing.
I've had a few trackballs in worse condition than that one appears to be that worked fine.
If you want it to work like NEW, then go ahead and buy the roller/bearing assemblies from Bob Roberts.