It seems to me you are going to want to use a local Database of some type, and it is going to have to be a file based one similar to access. Any other way is going to take the user installing a database program and configuring it properly which is going to be a nightmare trying to help people with different operating systems,etc.
If it is an online data base: users x different seeds x visited sectors x number of items x number of mod users = face palm.
It should support sql queries, or the like. Ideally, you need to save the seed, which will be the unique I'd for the current game. You are also going to want to store sector coordinates for range searches.