Borgin and Burkes is located at 13B, Knockturn Alley. It sells objects and artefacts of Dark Magic. The shop is old and murky inside. Shelves holding dark artefacts and objects line the walls, glass cases show off jewlery and more befouled dark magic objects. Here witches and wizards can buy or sell dark magic artefacts, and the owner is currently in look for a clerk to help with the shop. Brogin and Burkes does not only sell artefacts, but also some potions not commonly encountered by common Apothecaries, this also goes the same for books.
There are many items For Sale at Borgin and Burkes. The oldest, darkest, and most dangerous artefacts are kept in the basement, which is strictly off limits to anyone but Borgin himself - indeed Borgin spends most of his time down there, doing no-one-knows-what. Look around and you may find what you desire.
Look around, or ask. Even if you don't see what you're looking for on the For Sale page, inquire and Borgin will see what he can do for you. Place your word bubble in the Role-Play section, at the top.
I've started adding conversion-receipts (ooc) when you buy/are considering buying something, just so that people get more of an accurate idea of how much things cost. These are, however, taken from 2001 when JKR said that a galleon was equal to five pounds, so don't take offense if they're a little off!