Would a voluntary fee work?

I wouldn't even recommend an amount. I know of a few companies that have done a "how much would you pay for this week" and the results were surprisingly positive.

Also, look at services like flattr, it's well suitable to easy micropayments.

If, as you say, charging directly isn't an option. I'd look at an open, voluntary donation. I've seen a few sites that have combined a "buy me a coffee" type donation button with adsense or similar ads, and they seem to be doing ok on it. I'd say so long as restraint is shown on quantity and placement of ad blocks it needn't detract from the experience. Even ebay and amazon have advertising on now.

You've also got the option of charging for a premium listing or similar - though I'd not be surprised if that ended up hurting the amount of voluntary donations.

I've also seen one service (I forget who) that is free to individuals, but voluntary subscription is about $5 monthly. You can then adjust down the sub you're willing to pay with a neat little slider in 50c steps all the way down to 0, along with a :) that steps down to :( So they're adding a little pressure to hint at the preferred rate, but gently.

Ok, I'm going to go a bit on a rant, but I'll also provide some suggestions.

First of all, why is going to VCs your first option for making money? That's not going to solve the problem of your business making money. I would avoid that for a multitude of reasons, first of which would be: you want to be focused on making money starting now. You want to sweat about it, have nightmares about it. Tweak things, test things, figure out what works that will make it profitable to run. The only problem that you solve by taking outside money, is that making money is no longer a priority for the business. You have a lot of runway and a lot of time to figure out how to make money later. That's not really ideal. And what good does it do the users if it has to close down because it can't make any money?

Now for some suggestions.

You say charging the listers is not an option and the site is used frequently. That means you've got power users, that would probably benefit from some premium features. Charge money. Find things that are not essential for regular users of the service, that some listers would benefit from. Don't underestimate people's willingness to pay for useful things. And don't be afraid of charging money. The thing with people paying for stuff is that they complain when it sucks. They give you feedback. If it's free it's just an inconvenience and they probably can't be bothered to send feedback.

It seems to me like you already decided that charging is not an option. Don't dismiss it just yet. You can still keep the majority of features free, and just charge on corner cases. View it as barter for value.

Regarding the donation, what works best is not having a fixed amount. If I'm really happy that I've bought from your site the one thing I've searched for in desperation and couldn't find anywhere else, I might go on a limb and donate according to my feeling of the contribution you've made.

There are countless paths you can take to figuring out how to make it profitable. My main point is though, that you won't have to think about it if you take outside money. Because it won't hurt. Not for the next x months/years. But the problem doesn't go away. You're just postponing it. And it's actually harder to mess with pricing later on. Because you'se set out some expectations. Users react badly when free things go away. If they pay for things from the beginning, it's a different mindset.

I think that the voluntary fee would work better if the listers perceived to have the same goal as you: Growing the site and getting more viewers for their listings. Make them feel like a part of your business and also see that without your business, theirs would not run as well.