I always personally try to sell as "turnkey" as possible parts so as to avoid putting extra burden on the buyer to have to source extra stuff.
I do typically give the buyer the option of purchasing new o-rings or not, in case they want to save a few bucks. But I think it's only right to offer new o-rings.
Nothing sucks more than used soft parts.
Would you buy used lower control arms and re-use the bushings on them?