The trouble with eBay is that people aren't habitually looking for Peter Bellamy artworks (they might be looking for Peter Bellamy vinyl, but not necessarily even think of looking for visual art). You have to make sure the right people know about it.

It's a shame someone couldn't organise an auction at, say, Cecil Sharp House, where it would become an actual event?

Failing that, I would decide on a (high) price and perhaps place it on Gumtree, then let people know about it. My hunch, based purely on intuition would be somewhere between £500 and £1000? Only because a lot of folkies are hard up. I suppose a wealth fan of both Bellamy and Bird might go higher...