Can you please consider the option to implement market offer upkeep tax.
It can be really small 0.01% or even less per day.
Best option would be to charge the player on day change and when posting the offer.
Normal regular players would not argue much about such tax but the 999999 for one gold or 999 for one Q1 weapon would disappear.
Well the players that leave lots of items slightly above the market prices just to dampen demand spikes will not be happy because this will cost them something and over long time this might be a fortune.
That way the market will be more stressed and I will not leave lot of Q1 houses at 1500 but just few or at reasonable price.
If the database back end is using numbers with sufficient resolution this might be not hard to implement. If there should be change in the data type, than probably it is not so good idea.