As with any purchase from an online auction site, safety depends on the integrity of the seller and on your understanding of what is really being sold. Beyond that, I think online auctions are an excellent way to get wonderful building materials and fixtures at bargain prices.
If colors are important, make sure you know the names of manufacturers’ colors and what they look like in the real world. I once had a car in a color called “Fire Mist Faun Metallic.” What color is that?
Do you think a computer monitor would do it justice? If sinks came in Fire Mist Faun Metallic, I might have to see a real sink before I ordered one to match my bathroom tile. Home improvement superstores are terrific places for that research. They always have a lot of different models and even more paint chips for special order items. You can look around yourself, and do not have to feel guilty about wasting a salesperson’s time. Just be careful when trying to match colors with online purchases.
Another thing you should know well is model numbers. One digit difference in the model numbers, between something you saw in a store and something you buy online, could mean very dramatic differences in features. Or, it could be something completely meaningless, like a newer model microwave with 1,200 watts of power instead of 1,175 watts.
Finally, make sure you know the dimensions of the item you are buying. For appliances, you can usually go to the manufacturer’s website to obtain that information, even for older models. Faucet sets generally separate into center hole or two hole (meaning one control for the water, or separate handles for hot and cold) models and categories depending on the spacing between the holes. This is important if you want to put a new faucet on an existing sink. For light fixtures, you need to know the obvious dimensions, plus the weight. Unless you have a sturdy beam onto which you can screw an electrical box, some fixtures will be too heavy for your ceiling.
When buying wallpaper, make sure all rolls are from the same dye lot; otherwise, the colors on different rolls might not match exactly. Do the same thing with carpet tile squares.
In general, make sure you are buying from a reputable seller, and make sure you know what you are buying. With those two thoughts in mind, happy hunting for online construction materials, and enjoy your significant savings.