Ok, so if I understand what you are asking for here. You want to have a single product range, that when you access it via a particular seller, you get a price for him?
So for example:
If I buy from acme, it will cost me £29.99
If I buy from BobsEmporium, it will cost me £14.99
If thats the case then would it not be more sensible to setup the each vendor on their own website (multi site install) and then you could get away with using default functionality?
If for some reason thats not a viable business logic, then when loading a product in adminhtml, you would need to observe the load of the product, insert the price based on the product + admin logged in, then save this information to a price table.
On the front end of the website, you would do the reverse logic: Identify vendor from the URL or other means, then load price into model based on that combination.
From there it should be possible to use default core again.