The legal situation in two sentences
The ruling of the European Court of Justice C-128/11 of 03/07/2012 extended the "principle of exhaustion" to software delivered digitally: anyone who lawfully acquires a licence may resell it. Conditions: lawful initial placing on the market within the EU, proper deletion at the previous owner, and a seamless proof of purchase.
An analogous legal situation applies in Switzerland under Art. 12 para. 2 of the Copyright Act (URG), likewise the principle of exhaustion for copies of works placed on the market. This is not a "grey area", it is practice secured by the highest courts.
What you should nevertheless check as a buyer
- VAT invoice as proof of purchase. Mandatory. Without a PDF invoice showing VAT, there is no business record and no audit proof.
- The seller is a real, verifiable company. More important than legal buzzwords: Does the provider have a public commercial register entry and a valid VAT ID? You can check this yourself in two minutes. Check the dealers below yourself.
- A real phone number + reachable support. Anonymous shops with no reachability are the biggest risk signal.
- Prices within a realistic range. If a Microsoft Office key costs CHF 19, it cannot be sourced from legal EU/CH channels, almost always third-country goods.