Customer reviews are a key factor in determining trustworthiness. Reviews are collected from Google, eBay, Chrono24, to name a few.
Another plus point is if the online shop is a Trusted Shops. Trusted Shops costs the shop operator money. They set conditions for the shop. Both during the initial qualification and at regular intervals.
The following will be checked:
Identity & accessibility: Is the shop quickly accessible by phone/email
Security & privacy: personal data is protected
Cancellation & return: complete and permissible legal wording
Products & costs: clear, understandable description and display of delivery costs
Ordering & transparency: the process is checked with test orders
Service & shopping experience: positive customer experience, i.e. the shop must be rated at least 3.0 by customers.
Another important criterion is that the shop's imprint includes the name of a person in charge or the owner, who you can also google and who is "real". A landline telephone number and/or a store are also positive features.