A few reasons why not to use rock from someone elses tank.
There's a risk that you could introduce some unwelcome hitchhikers, but this risk also exists with fresh
LR.
As rock ages in a tank it loses some of the diversity it may have had when it was fresh, this can be overcome by also adding some some fresh
LR in your tank as well. Small portions of
LR should be swapped out for new every once in awhile anyway to maintain diversity.
If the aquarist didnt properly care for their tank the rock could also have been subject to
phosphate buildup which could result in nuisance algae. If the rock is covered in nuisance algae you may want to pass on it, or you could follow a process called rock cooking which is a method of removeing
detritus/nutrient buildup from within the rock.
On the plus side the rock is already cured, it should be available at a lower cost than new.
I probably wouldnt hesitate to purchase used
LR from another aquarists tank. Make sure to properly quarantine it first as you should do with every addition to your tank.
