Just as you said, they are auction houses. Most do not have Moderns, let alone Modern variants. Also, items typically cannot be purchased when I want. I am at their mercy for payment and shipping schedules with no recourse except not to buy from them again. I do not have the option to barter with an auction house. For example, I'm not sure why Metropolis has a "make offer" button. Every time I've tried that they say something to the effect of "sorry, that's a hard price" or "that's consignment, can negotiate".