A block of emerald, which composed of 9 emeralds, will make a merchant stay in the world, but why don't you just use only 1 emerald to trade with him and have him stay?

Also merchant don't despawn after traded once

Because then you would simply trade once and flood your world with merchants that never leave. These villagers are designed to be roaming, not to stay. So if you want them to stay; you will have to pay ;)

Merchant also trade for nametag, and IMO that work much better than the block

What I said, is that, in this current version, merchant will stay after they are traded once, so only 1 emerald is enough to have then stay

Huh? That can't be possible; the traveling merchant clearly checks for a despawn and there is no code connected to doing a trade. That's not a part of my mod at least.