I only play as a online player. If the people who are playing in offline mode had all equipment and items bound to their character and zero trades, including buying/selling to NPCs whilst online, it would be best to try and avoid economy issues online. But then again if you are playing with another friend who is a pure online character and you play with your over powered offline character, you could end up BOOSTING online players making it unbalanced again, then I can see other players turning it into a online business to exploit power ups for 1hr for $10! Seen it in so many games.
Only alternative to allow offline characters online would be dedicated server option with offline characters only, again no buying/selling/trading possible. The problem I could see however, is people claiming to of lost stuff from going from offline to online mode and that causing a whole other issue.

All of those are good points, and I get what you're saying, but I don't know of any games that have community made 'services' like what you're referring to. What were the games you played where players actually did that? There isn't really anything you can do about boosting to begin with. People could just make an online character and their friend could just be starting the game. There is no way to be able to prevent that. The whole trading thing could be managed to not ruin the game by not making a trading board, but instead by making it on the official website like PoE or some other website, although PoE does have more items that are considered 'currency'. However, the whole trading post or board is a bad idea to begin with. Just ask any Diablo 3 player that played when the game came out. They trashed it because the prices for the items were being inflated due to one player selling items at prices no one could ever buy. That basically ruin the in-game economy. I understand the boosting argument, but there is nothing that can be done about that. Something like that is inevitable in games like these. They are bringing trading into the game, and I hope they do it right.