Basically this. 
I middleman pretty often. I currently have six different stacks of cards on my desk for six different people I’m middlemanning for.
Should one of them stop responding to my messages for some reason for a long period of time, the cards will simply collect dust on my desk. 
In fact, I have an seventh stack of cards on my desk that I’m not middlemanning, but rather are duplicated cards I reserved for someone to possibly sell to, which did indeed ghost me soon after and I hadn’t heard from for almost a year, until recently. And those are still reserved.
Of course, if I was actively selling stuff, or Ralts cards would be worth a lot and therefore worth selling in general, I’m not sure I would have kept them reserved for years. 
But with cards I’m middlemanning for I defintely would. They’re not my cards. I would consider myself simply a warehouse. And although most middleman service websites have a limited hold time of about 30-60 days until you’d have to start paying money for each additional day, and they’ll discard the products after ~75-100 days, I personally have no issue keeping cards for multiple years. Cards don’t take up too much space anyway.
Greetz,
Quuador