I also suspect that the delivery times are too long - so many people want things quickly.

When we started selling on eBay we had some key criteria we wanted:

1, Readily available

2. Low price but reasonable margins

3. Easy to post both in terms of size and fragility

4. Something we could hold in stock in sufficient volumes to meet sales, with short supply times.

5.  Some key items that provide a point of difference.

Other people above have mentioned other factors such as key words and presentation. You also need a range of items (having 100 items makes you more visible than if you had 10).

The other ingredient is patience ... it can take time for things to happen ... and it might never happen.