A market listing has three parts as far as I know, M A D, with M being the listing ID and A being the assetid (D always provided)

An example of a non-populated (can be found in regular inventory response) steam://rungame/730/76561202255233023/+csgo_econ_action_preview%20M%listingid%A%assetid%D910550409944203216

And one populated: steam://rungame/730/76561202255233023/+csgo_econ_action_preview%20M5231585837963410495A36933563414D14870894776049808240

This populated one of course returns the float and other values you're looking for (in this case 0.31265467405319214), I utilized CSInventoryAPI.com for that.

Can you share a little more about the data returned about each marketplace item? That would make it easier to help : )