Jeff,

For a start, may I suggest you put up a PayPal Donate button. Simple & quick - get the code from PayPal's website.
Ask users to donate if they successfully sell a watch on here, a small percentage.
If I sold a $1000 (or any amount) watch without paying any ebay fees etc, I'd very happily donate.

Once a year you could have a donate-athon, to try to raise the server costs for the year.
Could even have a prize giveaway - randomly picked a donator out of a hat wins something nice - like a watch book, Heuer baseball cap, Heuer patch cap, watch strap, Heuer buckle etc etc. (one of them, not all of them!!)

The beauty of the above is that it is little hassle.

All the best
Stewart