Resident one decides to leave a town, their plot is left behind.
The plot is automatically set to sale, so any resident in town could potentially get it.
We in town x don't allow it. We're closed and private, and we assign which plots are available to players.
So I /plot nfs these plots and are free to put it back up for sale again.

But, I do not read in the town application form the following situation

- what is the policy in regards to placed blocks and suchs on the plots that are left behind by the resident?
   if there is a house, and we put the plot up for sale - whatever is in the plot become property of the new resident?
- are we to clear the plot manually (remove a house), if so, what happens to the items?

And finally, i can imagine a resident doesn't know they wont have access to the plot or chests anymore, what's the best situation, just make it embasy and let them take it and then ask them to unclaim?

But.. what if someone already bought the plot (say it's been 24h or a couple of days)

But what about the items that despawn, surely the town isn't responsible for being unable to pick up all the items in a /plot clear event. Some have so many that it's just despawning before we get to it all.

If they despawn that's the same as deleting them. What I do is I suck up an inventory full and just delete the items, fill up inventory, delete items, and so on and so forth.

I just keep going back and forth until everything is picked up. Sometimes I'll put it somewhere temporarily until I can organize it or put it where I would like it to go.

No I already answered your question. Place a chest in the area in which you'll put those items, even if it's temporary, and keep going back and forth picking up all the items until everything is picked up. I do this all the time and nothing despawns. Once all items are picked up then you can sort them where you'd like permanently.

I don't use any special mod powers...I just do the same as anyone else.