I always refuse helping lowbies if I play an evil. It makes zero
sense for precisely the reasons Daevryn stated.

As for getting help from the lowbies. It will usually come for free. Lots of lowbies scout on their own accord in the hopes of getting some loot from high level PK.

As for selling to lowbies. I don't hand over any items before having received the cash. That being said, I usually part with the item afterwards.

From the lowbie perspective, it works like spam email. You spam enough heroes and one of them might be bored/whatever enough to help you. That's fine, but I se no reason to encourage that kind of behaviour.