I'd actually get the used one, as Crypto Currency miners only target High-end GPU's. Low end GPU's like the 1050 TI when purchased used are most likely going to come from a realistic individual.
On top of that, GPUs tend to run smooth for several years without issue. Especially on the Nvidia side.
So, you can call it a gamble and a risk by purchasing the used one. But, there really aint much of a risk at all.
Make sure he/she has pictures of the GPU and they claim it is 100% working before purchase.