The Booksets website (link above) sells more than just second-hand books (though it sells a lot of those). From the homepage: we have a "huge stock of specialist extracts, articles, and offprints from hundreds of journals dated between 1650 and 1950. We have a terrific selection of Victorian material from the Edinburgh Review, the Dublin Review, and Blackwood’s Magazine. "
If you want an act of parliament, a pamphlet or antiquarian book, this seems to be the place to go. The company is UK based, but its international postage rates are extremely reasonable — including discounts for multi-orders. One of my bugbears with Amazon is that if you order more than one item from a third-party seller, you have to pay the same postage (plus the Amazon cut) for each item.
The Booksets site has a good search engine (search by author, title, description or ISBN), as well as browsing by category.
There is a sister site called BooksetsExtra which sells discounted new books, cds, dvds "not featured on our traditional Booksets site".