Transmission Fluid

welcome! it's not required, and i doubt it's dirty. that being said, some veterans here feel it's a good idea every so often. you can do it yourself, and it's quite easy. there are plenty of threads here with instructions and pictures. how much do they want to do it? a decent price is around $100. all the best!

 

Since it is used, you really have no idea what the previous owner did or did not do unless they had the service done at a toyota dealer, then you can look here: Sign-In (you have to register your VIN to see history)

Toyota has no scheduled interval to change the ATF WS, I personally recommend 30,000 miles, then 90,000 miles, then every 90,000 miles, but I am just some guy on the internet.

Prius C Tranmission/Transaxle/HSD Oil Change | PriusChat

(My dealer did my ATF change for $90, then $110, then $90. so that is a good neighborhood if you fall for your dealer's pressure. I would get a second opinion)