There has recently been an influx of scamming and sharking. For your own benefit if you have the slightest doubt in regards to the trade, ask an experienced trader for advice.

Also there have been a number of successful attempts of quick-switching in trade. Quick-switching is the act of intentionally substituting the item in the trade box for an identical item, nut of a different quality, hoping the other person doesn't realise and clicks trade, E.g. quick-switching a strange with a normal item. This is especially occuring when trade is being unstable and crashing, at this point patience is wearing thin and you may forget to check the item.

So always check what you're receiving and whether it's a fair trade, or risk being scammed.