When people trade in the anime, they close by. Why can they just trade pokeballs.
Dawn (anime) takes the pokeball from Ash (anime) and she gives her "old" pokeball.
There's no need for a big machine to do it for them. Please help me! Thank you! :)
Note: Since I am a huge fan of Dawn, I will being using her in most (if not all) of my example. (If revalent.)
This Is My Theory
They have trade machines because ,
1. To evolve Pokemon that evolve from trade, for example professor junipers and Bianca's trade of shelmet and karrablast.
2. I think it officially completes the trade and therefore ownership is decided. For example if they do it by just exchanging poke balls they can claim that the other party stole his/her Pokemon. This will prevent Pokemon theft, and legal action can be taken in the anime if theft takes place.(team rocket lol)
3. The anime makers wanted it to b more like the game.
This is what I think the reasons are, but no on except the creators know for sure.
Hope I helped :)