It is fine to have a used deck. Many of mine are.
One thing I do when I get them is to sort them all into order.
This makes sure that all the cards are present.
It also feels that I am alligning them for a fresh start.
I have a particular order which I sort them into.
From the top
Majors, Zero to XXI
Wands, Ace to 10, Court cards ending with the king. (or knight for thoth)
Cups, ditto
Swords, ditto
Discs, ditto
Then I shuffle them, then sort them again.
And so on.
At some point I choose the 'first card'. This is my introduction to this deck, it is suppose to be a sort of 'hello'. And an indication of how my relationship with this deck will be.
You may find a deck that has a strong feeling about it (either good or bad).
This can usually be removed with a cleansing ritual.
Sometimes I just talk to the cards, tell them they are mine now.
Some decks have a smell about them. Sometimes incence, sometimes musty.
I find incence nice, and musty can be removed with more incence.
You can get loads of good decks cheap, second hand.
It's a great idea.