Replace inner sleeves on used records?

For maximum value yes.

Only if you care about resale value but if you do want to keep them then the Nagaoka liners make a decent alternative to the complete inner sleeves of MoFi and Goldring and will fit inside the sort of original liners that have lyrics or pictures on them.

Old record sleeves can have dirt and debris that can scratch vinyl when you slide them in and out.  Many new releases have those hard, almost cardboard sleeves that can also cause rub marks against the record.

Unless a plain white paper inner sleeve, don't throw away the original. If there's room in the jacket - and there usually is - just put the unused sleeve back in and don't use it. Put the open end of the sleeve facing up in the jacket so it doesn't get in the way when inserting the record back in the cover with its new plastic sleeve

what @slaw said and historic value.