Good question, but who can say? Surely something is only worth what others are prepared to pay for it? I would say some "collectable" items, like old comics or toys are lucrative. Pippa dolls especially can go for �100+ if in good condition. Retro stuff. But things usually go high ONLY if there are 2 people bidding against each other, which pushes the price up.
The $1,000,000 question! I have sold a few things on e-bay. Some didn't sell at all, some sold for 99p and then I've had things which have sold really well. I sold a board game based on rugby for �25 during the rugby world cup, a tourist guide book for Guernsey (pre WWII) went for �46 (I'd paid 50p for it!) and what I thought was an unremarkable postcard of a town in Cornwall sold for �13!
I would say something you can reproduce without needing help from others. i.e a skill for woodcraft, selling name plates, birdhouses and the like, however e-books tend to sell ok if the price is right.
I had a clear out a few years ago and sold all my old clothes, golf clubs and daft stuff i didnt want and i made �850 just from old rubbish, but what is one mans rubbish is another mans treasure.