Hey there! That's a really smart question - buying industrial PLCs on eBay can be a minefield. Let me break down how to spot what you're really getting and the risks involved.

First, for identifying what you're actually buying:

1. Check the seller's reputation - Look for industrial automation specialists, not general electronics resellers. Check their feedback history for similar PLC sales.

2. Ask for provenance - Where did this 'new old stock' come from? Legitimate NOS should have documentation showing it's been in storage, not pulled from a working system.

3. Examine photos carefully - Look for signs of use: scratches on connectors, worn labels, dust in crevices. Genuine NOS should look pristine.

4. Check the firmware - If possible, ask the seller to show the firmware version. Counterfeit units often have mismatched or outdated firmware.

5. Price comparison - If it's 'too good to be true' compared to authorized distributors, it probably is.

Now for the legal/liability implications - this is serious stuff:

Safety liability - If a counterfeit or refurbished PLC fails in an industrial setting, you could be liable for any injuries, property damage, or production losses. Industrial equipment has strict safety standards.

No manufacturer support - Siemens won't provide technical support or warranty for unauthorized products. If something goes wrong, you're on your own.

Cybersecurity risks - Counterfeit PLCs could have malware or backdoors that compromise your entire industrial network.

Regulatory compliance issues - Many industries (pharma, food processing, energy) require certified equipment. Using unauthorized gear could violate regulations.

Intellectual property violations - There have been cases where counterfeiters were ordered to pay millions in damages for trademark infringement.

My advice? If this is for anything safety-critical or production-critical, buy from authorized distributors. For less critical applications, at least buy from reputable industrial automation resellers who provide warranties and documentation. The few hundred dollars you might save on eBay isn't worth the potential liability if something goes wrong!