"F" in the Relic Hunters A to Z is "Flat Broke", whch is what most Relic Hunters perpetually seem to be. I thought about talking about player motivation, but instead I'm going to share a valuable lesson I learned about getting buy-in from your players when you want to do something horrible to them.
You're probably familiar with it from one side of the table or the other: the DM has a great idea—or a published adventure—but it starts with the players being imprisoned or kidnapped or having all their stuff stolen. And as everyone knows, players will fight like hell to prevent any of that from happening. It doesn't matter that it happened to Conan or John Carter with fair regularity, it's by gods not happening to them.
Some DMs will just try to power through it: overwhelming odds (which leads to either an upset victory or a TPK), cheesy tactics or abilities that will never show up again ("uh…no, you get no save against the sleep poison. It's…special, yeah." or just plain old DM fiat. Some games have a method to bribe players to go along with it—players get plot/action/hero/whatever points, which takes a bit of the sting out of it, but it still feels like they're being railroaded.
The first thing to keep in mind is that this sort of plot is a terrible idea if you don't have your players' trust already. I wouldn't try it on a new group (unless it was somehow pitched as the basis of the campaign: "you all start as slaves" or something) and I wouldn't try it on a group that was already having problems with their DM. If you've got that kind of situation you don't need the added stress.
The second thing to keep in mind is that players, by and large, want their characters to be cool. Taking them down or making them look like chumps completely undermines that image. Some people would rather abandon a character than have that taken away.
When I tried to pull this stunt in my own D&D game several years ago, I got it half right. I went to the players to get their buy-in first: "Hey guys, I've got this idea for how I want to take the campaign, but it kicks off with you getting captured." Well, there were some nervous grumblings, but the first part of the campaign had been fun and they trusted me to continue to make with the good stuff.
But then I put forth how I thought it might happen (drugged wine at a tavern while pursuing a lead, as I recall), and that was a mistake. Nobody thought I was doing justice to their character. So instead I said the five best words I think a DM can say:
"Tell me how it happened."
Instead of the relatively boring drugged wine, we got an attempted poisoning, an ambush, a desperate attempt to escape, a tavern set ablaze. I didn't have to railroad or DM fiat a thing, the players were satisfied with both the plausibility of the kidnapping and their character's response to it, and we still wound up where we needed to be.
So, going back to Relic Hunters: after every piece of downtime, I generally plan to start with this: "You're flat broke. Tell me why."
It sets the characters up with a reason to go after the next big score and it gives them an opportunity to say something about their character. Someone who blew the cash gambling or on ale & whores is a different kind of person than someone who packaged it all up and sent it to their ailing mother or is trying to pay off a debt.