That just looks like an incredibly crude attempt by a scammer to make a virtually worthless postcard look like something worth a fortune!
The postcards were clearly sent from Oban and Gateshead, to addresses in Leicester and London respectively, so there would be no requirement for them to ever get near to the Titanic. Further, the second one is clearly dated April 10th 2011, which is before the ship was launched in May of that year (and exactly a year, to the day, before she set sail on her maiden voyage).
Anything passing through the Titanic's mail room would probably have been franked by a small rubber stamp, rather than having a hefty great solid 'postmark', which looks exactly like the output from an inkjet printer. Further, it's more likely that they would have used the word 'aboard', rather than 'on board', as the (US) National Postal Museum at the Smithsonian Institution did in the title of their 2001 exhibition:
https://postalmuseum.si.edu/fireandice/titanicsmailclerks.html