I'm quite sceptical of getting second hand GPU. Or are they selling them refurnished?
As long as they werent used for Bitcoin mining there is no reason to be skeptical. Now, if it was a
CPU thats a different story. Most people take a GPU as it is and stick it in their PC without doing any aftermarket changes. As most GPU's come with great coolers including the NVidia stock one, its the PC part that is the least prone to wear overtime due to like dust etc. if someone hasn't kept their PC well cooled. However something like a CPU and RAM can eventually have problems.
Not to mention even if you spend 200 on a 980Ti and it lasts you 2 years, thats worth it in my opinion. I bought my 680 second hand and it lasted me 2 and a half years.
Also refurbished means nothing. If a card is damaged internally then its damaged. Refurbishing would be to change the shroud, clean it etc. Like buying a refurbished Laptop, they would replace the keyboard, maybe the screen if need be then thats it.