Being that a money changer is expert he isn't liable for a mistake. The Gemara is clear that it is a situation where the money changer had no reason to think he may wrong.Therefore if he is, it is considered beyond his control so he has no liability.
A guy on the street who doesn't know so much about coins shouldn't be offering his uneducated opinions and potentially misleading others to begin with. Therefore if he is wrong he is liable.
The Gemara is not referring to a situation where the money changer was getting paid for his opinion.
The Rambam Hilchos Sechirus 10:5 says as follows
הַמַּרְאֶה דִּינָר לְשֻׁלְחָנִי וְאָמַר לוֹ יָפֶה הוּא וְנִמְצָא רַע אִם בְּשָׂכָר רָאָהוּ חַיָּב לְשַׁלֵּם אַף עַל פִּי שֶׁהוּא בָּקִי וְאֵינוֹ צָרִיךְ לְהִתְלַמֵּד. וְאִם בְּחִנָּם רָאָהוּ פָּטוּר וְהוּא שֶׁיִּהְיֶה בָּקִי שֶׁאֵינוֹ צָרִיךְ לְהִתְלַמֵּד. וְאִם אֵינוֹ בָּקִי חַיָּב לְשַׁלֵּם אַף עַל פִּי שֶׁהוּא בְּחִנָּם וְהוּא שֶׁיֹּאמַר לַשֻּׁלְחָנִי עָלֶיךָ אֲנִי סוֹמֵךְ אוֹ שֶׁהָיוּ הַדְּבָרִים מַרְאִין שֶׁהוּא סוֹמֵךְ עַל רְאִיָּתוֹ וְלֹא יַרְאֶה לַאֲחֵרִים.
When a person shows a coin to a money changer and he says that it is acceptable for use, and it is discovered to be unacceptable, if he charged for his services, he is obligated to pay even though he is an expert and does not require further training. If he did not charge, he is not liable, provided he is an expert and does not require further training.
If he is not an expert, he must reimburse the questioner even when he does not charge for his services.
The above applies when the questioner tells the money changer: "I am relying upon you," or it is obvious from the situation that he is relying on his opinion and is not seeking another opinion