If you check the Citizen newspaper, they are called MacGregor's auction house and they put a box ad in the classifieds every two weeks when the auction is on. You can go during the day on a wednesday to view items. They auction small items on a Thursday and furniture etc on a Friday. You can either bid in person, or simply write your highest bids on a form you collect from the desk and they will follow your bids from the paper. You can call to check later if you won any auctions.
You do pay in cash but you only pay when you are picking up your items to take them away. You have to pick them up by the tuesday following the auction. The place is on Tom Morris Drive. You can phone 01334 472431 to find out auction days too.
Also, for other items you need, you could try the listings on Dundee.gumtree.com for things in the local area. You can also get lovely vintage items in Rummage on South Street, near the westport arch.