First of all, I'd prefer to get rid of the account based guilds and 5 limit. Instead I suggest 1 guild per character, for that character only. I also think guilds should be limited to one faction, but I haven't played and don't know how that would affect the economy.

Now onto GAHs, being limited to one guild would naturally limit your market, so I suggest a little rework.

Instead of only having access to GAHs that you are a part of, you could browse them at certain areas. These might be guild halls, stalls, or just high ranking members of the guild(or something else entirely these are just examples).

As it stands now, being in a guild on one character, means being in that guild for all of your characters, from what I understand. I'm not fond of such a system since some guilds might be a good fit for one of my characters but not another. in addition sometimes I like to have guildless alts to play without being messaged.

Overall this would still encourage trading between guilds and such, but is a bit less limiting.