Three observations about the argument at Republic I, 348b-350c:
1. Plato implicitly asserts that a just person has pleonexia. If a just person 'wishes to have more' only as regards an unjust person, then in some circumstances he has pleonexia.I take it that we should view Aristotle's doctrine, in Nic. Eth. V.1-3 (= EE IV.1-3) , as a deliberate correction of each of these points:
2. Plato implicitly takes the standard of pleonexia to be absolute or strict equality. Since a just person wishes to have more than an unjust person would have, then the relevant standard, in relation to which he has more, is that he have the same. (So, for example, if it were right for a just person in a distribution to get 60%, when an unjust person, in those circumstances, should get the lesser amount of 30%, then, even though this is just and right, still, the just person would be getting 'more'--because it is more than 50%.)
3. Pleonexia is shown toward actions, especially virtuous and noble actions, and not only as regards 'commutable goods': to look for more of a say; or to have greater authority; or to have a greater claim to decide how something should be done--these are all instances of pleonexia.
1'. Pleonexia becomes a mark of a certain sort of unjust character; a virtuous person in no way displays it (1129a32).
2'. Pleonexia is decided relative to proportionate equality (1131a22); thus to want what is materially more, if you deserve more, is to want what strictly is the same amount.
3'. Pleonexia is properly for goods of fortune only (1129b2); it cannot be shown relative to such things as just actions.