This video should reassure you on transferring tickets on the app, I bought a ticket from somebody last Euros and it was straightforward, you just need to enter the email of the person who's buying it off you and click transfer.
https://www.youtube.com/watch?v=roGCRCcDAKc
P.S. The above is if you got your ticket through the public lottery, if you got it through the SSC the process of transferring your ticket is more complicated.