You forget about the "instant getification" factor.

If you're planning ahead, then yes, you would use the Buy setting on a retainer to try to get the materials/goods you want.

If your need is acute, and you don't have the luxury of hoping someone wants to sell to you at an offered price, then you will go to the MB and buy directly from someone else's retainer in Sell mode.

And believe it or not, with how quickly stuff can be undercut, a Buy order will likely be left up a while (say, overnight) and the price of the item sought will drop below the Buy order offer. At that point, some enterprising fellow can easily come along, buy the cheap item off the MB, then sell to the Buyer, acting as middleman and making a little profit.

I really don't see your concerns as being valid. Having both buyer and seller in other games works well, bots or no bots.