Suggestions

When user tries to auction stuff for gems, they have to manually select gems in addition of writing the desired price, if the currency is not selected, it will give a bit vague error report.

This is a problem because on daily basis people come to bug report forums complaining about auctions being broken, and it's always same issue: they did not select gems as the currency. Right now on the first page of bug reports alone is 5 different people reporting this, and since this spawns consistently pointless bug reports, it's pretty obvious that this should be changed.

So what I am suggesting that either the error message should be clearer (like just an addition that says "you may have forgotten to select correct currency, or you may have put non-numerical characters in the price field" would be enough) OR do not require selecting the currency at all (I still have no idea why it's required in the first place) OR make it so that by default currency is not selected at all, and if user tries to post an auction without selecting currency, it would deny it, and clearly state that currency was not selected.

I like the last option the most. It's straightforward, and no need to reword the no-one-ever-reads error message and the problem can be easily spotted by the individual.

Always in support of quality of life changes. Maybe do away with the currency type box altogether, and get a clear error message pop up if you try and enter prices in two different types of currency only?

I think it has to do with other sites that automagically update the selector when you enter things in the textbox. People don't check anymore because they're used to sites fixing this for them.

I prefer a better error message rather than an automagic switch to whichever currency you've last typed in the box for since people change their minds or just type in the wrong box. Remember all the "why did this sell for 1t?" bug reports?