To add to what Tayis said we also plan to have skills which could effect prices aswell as other aspects so maybe you'd want to save the higher valued items for when you have a certain skill active. Really its to add in a choice to strategize a bit. Fluctiating every price might be a bit to much atleast early game but perhaps later in the game progression there could be a market that changes a bit? Could maybe be a fun little extra mechanic to stratigieze haha
Oh also in this current build there is no chance system in place which is going add a bit of balance aswell. This means every item right now has the same chance of popping up as an item some one wants to buy and also the Miner could get any one of the items as they too have the same chance. In the future we will have all these stats set up so its a bit more of a strategy you'll have to play to make even more coins!