This is something we've been feeding back to Zenimax for a long time now (in fact, I recall highlighting this as a problem about 2 or 3 YEARS ago).
The guild trader system is a tough one - with no single point for a universal market to give everyone a chance. I'm okay with the premise of the guild trader system, but the implementation of it has been exploited now for far too long. Big trade guilds put the big bids in on the major cities - those are inevitable - but as they vie for a spot, they make fake guilds and put in very large bids in other trader rows (as insurance in case they don't get the spot they have bid on). Then, they either sell on that fake trader (if they win their main bids), and disband the guild - or they let it ride as a fake guild with no wares in it for a full week. This completely bottlenecks the market towards the major cities, and in turn prevents any diversity for the smaller trade guilds (which inevitably fold and players move away from the game).
The solution is a very simple one. If a guild wins the bid, they hold that trader for the week - even if they disband the guild, the trader remains blank. This means trade guilds no longer have the option to create a "backup" trader as insurance. It will mean more guilds winning bids and selling their wares - allowing far more players to be involved in Tamriel's economy and trade.
It isn't rocket science to implement, and I'm utterly shocked, and disappointed, that it's not been in place for 2 years now (since it was brought to the attention of ZoS back then).