In my opinion, the style of trading/selling that best fits this game depends on what the end game development it meant to look like and the target population. If you only expect to have 200 people online at a time, direct trading from character to character probably isn't viable. Also, if the distances are so great that to get that copper ingot from Bob you have to run 45 minutes, that won't work either. Regional AHs will only work if you have regions. Right now, we really don't. I am not sure the overall scope of the map nor what the reality of that will become. Also, our current chat system in game does not support a trade channel and if we have any significant number of players, trade spam will be so bad, not to mention gold spammers, that you will have a very hard time finding a trading partner for the services/goods you are seeking as is will scroll by too quickly. I do not in any way like the Path of Exile out of game sales method either. Also, with our limited inventory issues, you aren't going to want to travel a long distance filled with 5 or 6 items to sell and not be able to gather net items or materials along the way, especially as the map gets larger and the distance traveled increases. If you make the ability to sell/buy from other players too arduous, few will use the system and the economy will die and those that enjoy that aspect of a game will move on.
With that said, there seems to be a portion of the community that wants this to be a slow, plodding, unpleasant and low population game. Some plead for realism then throw that out the window when it doesn't support their new desire for a system in the game. To accurately and usefully assist in positive and beneficial suggestions/feedback/input to the developers, we really need to have a decent understanding of the game release targets with respect to desired map size and player population as these have a very significant impact on many things not the least of which is how to craft a functional economy and trade system.
If we have 1 city, a global and a regional AH is the same thing. An AH allows the developer to adjust market taxes/costs to help balance the in game cash flow and have a coinage sink. I like regional resources and, to a degree, so far we have that as resources are somewhat tiered to the mob levels they are around. That said, not sure how far one would have to go to go from T1 to T5 resources. That would determine if regional AHs would be feasible. Same goes for crafted items. You are likely to make crafted items where the tier of the materials are available for the items you are crafting. Now with nothing being BOP/BOE currently, that means a market for all items including the very rare named drops. The distribution of wanted named drops will determine, again, if regional AHs are reasonable or not. If you never have to travel any great distance to go from a T1 to a T5 resource, then regional AHs won't work and a global AH might as well be used.
If we do go with a regional AH, you could always allow purchasing remotely and requiring physically going to the location to pick up the purchased item. You could also allow a fee, hopefully distance based, to have a 'merchant caravan' deliver your purchase to the AH you chose to work out of. You could also pay a 'merchant caravan' to port your items to a distant AH and place them up for sale at the price you wish to sell them for. You could also allow personal vendors, or even 'market days' as some have mentioned and allow those to not have the AH fees and help focus more player to player interactions on those days to help bring that social feeling some have mentioned to the economy as well. Removing the AH fees with personal vendors might result in lower costs for the items or at least remove the overhead of selling on the AH and motivate more participation on those days. It will, of course, bypass the money sink aspect but the need to travel and potentially go vendor to vendor trying to find what you want may still make some folks prefer the AHs. And if the vendors are only on market days and not full time, then the AHs still would play a pivotal role.
I am a crafter/resource gathering/economy kind of player. These subjects are near and dear to me. I fully understand they aren't to all players. I think it is possible to devise a system that can be used by those that want it and ignored by those that don't or only use the parts a player wishes without ruining the game for anyone.