Hi Silver,
I am just starting my 2nd full season and like you learnt from youtube and this forum. So its definitely possible.
You tube is great for learning bee handling skills, plenty of vids on what and what not to do ;-). But has its shortcomings too.
I find the changeling bit to learning without a mentor, is all the conflicting advice you will receive. If you have a mentor, they will teach you "their" system, which when all put together works.
But now its up to you to pull various bits of info together to work out your own system. And when seeming honest and knowledgeable people are telling you to do different things, it can get confusing at times.
I know others have said it, but don't blame the guy who sold you the bees. Its impossible to know for sure why they died.
You said " i spent time picking them up and putting then on the landing board ". That sounds like a bad idea. Sometimes its the sick bees that are too week to make it back. Or in other cases infected bees will intentionally leave the hive to die.
"guarantees bug free" - I can sell you a bug free hive mate. Its currently flat-packed in my attic. As others have said every hive on the main land will have some mites, and if they don't they will soon. Other bee viruses are also endemic in the UK and found to some extent in all colonies.
Good luck with your new hives. And do not hesitate to ask if unsure about anything, there are some great guys and gals on here that I am sure will come swiftly to your aid.