On https://steem-engine.com/?p=faq:

Are there any trading fees?
There are no fees for using the Steem Engine market, however please note that as with anything on the Steem blockchain you will need to have enough Resource Credits available in your account to publish the required transactions. Also, as mentioned above, there is a 1% fee for any deposits or withdrawals of STEEM.

That last line is also reminded on https://steem-engine.com/?p=market&t=STARDUST near DEPOSIT/WITHDRAW buttons:

NOTE: There is a 1% fee on all deposits and withdrawals.

So basically, they say that trading (conversions) are free, but moving funds into trading account and taking them back out costs you ~2%.