Tag sales are generally safe. The only thing you have to be careful about is purchasing fake or stolen items... and bed bugs.
If you buy an item and it turns out to be stolen but you didn't know it at the time, then you can't be held liable for its theft.
If you suspect an item like a bag or pair of sunglasses may be fake, don't buy it. You should also consider telling the owner so they don't accidentally sell it as an original.