I'm a beginner in servers, and I was wondering if it would be possible to make a sell/buy system for lands, where players can build and do all the stuff they want while their land is protected, and at a specific time I can disable that land protection so people can build in other houses, steal their stuff, etc. and then re-enable it just like it was without having all people reclaiming their land Is this sort of thing possible?
To protect land at this point in time(1.8.8 is latest) you need to go beyond vanilla Minecraft and get a mod.
Installing mods requires Forge, which might mean falling back a few versions(probably to 1.7.10). It also means you and all your players will need to install forge and the mods you use.
The only area protection mod I know much about would be TerrirotyZone.It's requires setting the territories manually, but that should be fine if you don't have more than a dozen players. You might be able to find others if you search.
For enabling and disabling protection you could just move the script from it's proper folder and reload scripts to turn it off and do the opposite to re-enable it.