Re: The long trade sidequest
I think the only way it'd work is if you didn't tell your players about it beforehand.
In the first encounter, have part of the "treasure" be a mundane-ish piece of gear or something; it has to be something worth their time getting, though, but not something they'd want to immediately sell.
Soon thereafter, before they forget it, have someone else need it; he'll trade 'em something else for it, and that something else has to follow the same kind of rules, but slightly more interesting. After that, don't have them trade stuff all the time, but instead maybe once every couple of sessions or so. Eventually they may catch on, which would be cool, and if they don't, well, that could be even better.
The only problem could be if they just chuck the [Item] away and forget about it, in which case you've lost a sidequest. I'd just abandon it if that happened, though, so you could try it again if you wanted.
Re: The long trade sidequest
This works in video games mainly because the hero cannot discard the trade item, destroy it, consume it, sell it, use it for any other purpose, etc. There are dozens of things that could happen to any given trade item along the way, making a direct conversion to RPG impossible for this sort of "quest".
However, if each item is something that is both durable and useful, the players can reasonably be expected to hang on to them. When the next NPC comes along wanting a trade, there are a bunch of things you can do to "encourage" the players to make the trade. Maybe the item offered is superior (for the players) to the one they have. Maybe the current item has outlived its usefulness in the campaign. Or maybe the trade is forced (i.e., make the PCs an offer they can't refuse).