Buy 30, the one nearest the average is the most correct.
Sell the other 29 on eBay stating "checked against my other thermometres"

Seriously though, you probably mean you noticed a difference of 0.2 or 0.4 C, I suppose. 4 C is an enormous mistake and 2 C also.
A common fever termometre (they cost a few Euros) is probably exact within a few tenth of C (or it would be useless and wouldn't be sold as a medical aid) and should work very well for colour development.