Generally, I tend to follow the collectible tier list, either directly, or using the search function to look up the collectible I want to sell in the thread to see what other users have sold it for in the past! You can also try searching the TBT marketplace to see how much other people are selling stuff in their shops for. However, it is user made and collectible prices can be quite variable. In the end, it doesn't matter a collectible's tier or prior sales if the demand isn't there, I know some collectibles have gone down in demand and price lately, and some more.
If you're really not sure you could always list them requesting offers, but in that case I'd still advise searching approximate prices so you don't get lowballed, although I think people on here are pretty fair.