I have a few 'loose rules' I apply when buying what I hope/know is a collectible object.
First: it has to touch my heart in some way, sometimes its the cut of the glass, or style of the designer/creator. Other times its the touch, or the whole composition.
Second: if I'm buying it as a collectible I never by 'second rate' items - unless the age, or rarity, of the item allows for this. Don't imagine you can repair chipped items - items that can be restored and hold their value are OK - buy others only if you never intend to sell for a profit.
Third: if I'm on 'the hunt' I do lots of research. If I'm just looking I go with my 'gut feeling'. The more you 'hunt and collect' the better your 'gut feeling' gets.

