Only if you need the miles transferred into your account do you pay the airline's fee for transferring miles! Say if you have 135K miles of your own & need another 15K into there to redeem a 150K award.
In most cases, the FlightNuts traders operate like our late great Coupon Connection: you book a 25K ticket from your account in your Trading Partner's (TP) name for him/ her to fly & s/he books a 25K ticket from theirs in your name, so no transfer fee to points.com nor anyone else.
Main downside is that it's probably (pa)trolled by the same mileage police who raided Coupon Connection last month!