It's not a big problem for you if you do not have a Steam account. The first time you launched the AVA launcher via Steam, it would attempt to "link" the ijji/Aeria account. to you Steam account. (by linking, i mean Steam will auto-login for you, some sort of info is saved to allow that. So if you never had Steam in you life, then you don't need to worry about anything related to Steam accounts.
Aeria did the same Steam integration after the migration, so what was said above is still the same.
What do you mean by old ijji accounts? Like actual old ijji accounts, or accounts that went through ijji times? If you mean the former, then I rather not take any chance buying anything not migrated to Aeria. Since there is very limited access on ijji, you can't show much proof anyways.
As for the trade, I can't explain about Paypal and refund info since I don't have enough experience in these sort of things. But it's better to be really safe using an OMM.
Those with the buyers never go first mentality, people with some influence on this section (MPGH staff, well-known people, etc.) can usually be trusted. (NOTE: don't go by post count!) If both the traders can't seem to agree on who goes first, then go OMM. You rather be safe than sorry.