An uninstall and reinstall of TS3 probably wasn't necessary; the issue usually isn't due to anything in the game's program files. Rather, it's most often a corrupt file in the game folder, or sometimes the game calling for more RAM than is available. So once you've completed your reinstall, it's best to remove the entire TS3 game folder from Documents and try out building in a new save. If it works, you can copy one of your existing saves from the old game folder into the new one and mess around in build mode a bit, to see if the problem recurs.
While you're troubleshooting, be sure to delete the five cache files in your game folder each time you quit. They'll regenerate whenever you launch the game, and they should be deleted regularly, even in the absence of any game issues. For reference, those files are:
- CASPartCache.package
- compositorCache.package
- scriptCache.package
- simCompositorCache.package
- socialCache.package
If the new save lets you build but the existing one does not, it's a good idea to check the Task Manager to see how much RAM TS3 is using when you load the older save. Build/Buy demands more memory than live gameplay, so you might be going over the limit, especially if the lot has a large number of items and/or the world has progressed. TS3 is a 32-bit app and can only use ~3.7 GB at a time; anything over 3.5 GB is in the danger zone, where a temporary spike can cause a crash, or an Error 12 upon saving. If the Task Manager shows memory use in terms of a percentage, right-click on the RAM header to switch to absolute numbers.
If you still see crashes in Build/Buy with a new save in a clean folder (or you'd just like to be thorough), please run a dxdiag, paste the results at pastebin.com or another free site, and provide a link here. It would also be good to know which packs you have installed and which world(s) you're currently playing.
https://help.ea.com/en-us/help/pc/how-to-gather-dxdiag-information/
