Hey there! That's a really smart question - I'd be suspicious too when I see a 'last one in stock' deal on eBay for industrial equipment like a Siemens PLC. Here's what I'd look out for:

First, check the physical details closely. Genuine Siemens parts have crisp, clean printing on labels and components. Counterfeits often have blurry text, poor-quality logos, or slightly off colors. Look at the manufacturing icon/logo stamp - fake ones usually have logos that are out of alignment or poorly reproduced.

Also, be wary of sellers from China offering 'too good to be true' prices. These units often underperform and fail unexpectedly. Check the seller's feedback history specifically for electronics sales - look for patterns of complaints about fake or non-working items.

Now for the scary part - the hidden risks are serious:

1. Safety hazards: Counterfeit PLCs can compromise your entire automation system, potentially causing serious injuries or fatalities if safety functions fail.

2. No warranty or support: Grey market products come with zero warranty and Siemens won't support them if something goes wrong.

3. Unexpected failures: These units can fail on the spot or malfunction unpredictably, leading to costly downtime and production losses.

4. Electrical hazards: Counterfeit components can cause electrical shock, overheating, short circuits, and even fires.

5. Regulatory issues: Using non-certified equipment can violate safety standards and create liability problems.

My advice? If it seems suspicious, it probably is. The risks just aren't worth the potential savings. Stick with authorized distributors even if it costs more - your safety and system reliability depend on it!