Well, from what I can tell, it looks alright. I know that pen pH testers are used in laboratories for a quick check on acidity. They are not the most accurate, more of a quick thing. Lots of chemists carry one in the pocket of their lab coats.
Anyway, it is stated it has an accuracy of 0.1 . Which, I would imagine is alright. Are you comfortable with a possible difference of either the pH being 6.5 or 6.6 or 6.4? All three of those are the possibility. Instruments are also tended to be higher rated, so it's up to your discretion.
One thing I would like to add, it is claimed to have a glass electrode. They are very sensitive, easily broken, and difficult to find a replacement (in most cases). I wouldn't buy a used one unless it were in my hands. It could be cracked, scratched, broken, etc.
If I were buying one for myself, I would pick one up for twenty bucks.
BTW, I'm an undergrad professional chemistry student and have worked at the environmental chemistry department at UGA as a lab assistant, so I have a little bit of first hand knowledge.
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