iPhone has yellow spot and touchscreen wont work

You’ll need to try a replacement screen (LCD & Digitizer) to see if it eliminates the problem. Also, because this phone was sold as “requiring repair”, then it probably has more issues that just that one. This being an iPhone 6 Plus, it probably has Touch Disease as well.

My experience with “broken” phones (except for those from people you know personally) is that they have already been through the wringer and can be a bottomless pit of fun and giggles <sarcasm>. They do help you learn though :>)

Do a thorough visual inspection of the logic board, flexes and connectors. If it looks like it was already through a round of micro-soldering (missing shields, flux residue, missing components) then you likely won’t repair this just with modular parts. In that case, practice stripping it down and building it back up…or have it refunded by the auction site.