I've no experience of selling, but it's best not to leave in customisations that could catch out a non-expert buyer. We've had a couple of these cases lately, such as someone who tries to run an RUU (e.g. Gingerbread upgrade) and ends up with an unbootable phone because it had the CM7r2 hboot. A ROM that's partly on sd-ext (eg a Sense 2 or Sense 3 ROM) could also cause problems for someone who doesn't know about this stuff. So whether rooted or not I'd say make sure it's in a state where the buyer can run an RUU, do a factory reset or change the sd card without causing problems or needing expert knowledge.