By treasures, I mean pieces that look great, will last and will hold, or maybe even increase, their value if you want to resell them in the future.
Search for products from well-established, high-end design retailers: Liberty of London, Heals, Chaplins, Selfridges, Anthropologie and The Conran Shop, etc. All these stores specialise in expensive and generally well-built homewares and furniture, so if the product is genuine - and that's one thing I can't guarantee - it should be durable and to be able to bought for a fraction of its original price.
Search for products by up and coming designers that have recently been mentioned in the press. I bought a beautiful oak light fitting by Cornish-based designer Tom Raffield for less than a third of what it would have cost new. And, it was new, it was an unwanted gift still in its original packaging. Rory Dobner, Tom Dixon, Orla Keily and Donna Wilson are some of the other names I regularly scour the resale sites for.
Vintage shop fittings, traditional wooden flip-top school desks, and architects' plan drawers are other pieces to look out for. These original pieces are growing in demand at the moment and will rise in value. Don't pay hundreds for them though.
Any private sellers selling pieces by any of the designers or companies mentioned obviously have good and expensive taste so always remember to check out what else they are selling whilst you are browsing. More often than not, their priority is to get rid, rather than get a good price. Happy hunting!