In a sense, yes you can use addons to help you get more World of Warcraft Classic gold. However, they don't just give you the gold. You do need to do some work to earn it still.

The addons can do a wide range of things however when it comes to the Auction House and the Economy, the most common are ones that help you find loot, value it, scrap junk and deal with the auction house better. These addons are legal to use. If Blizzard announced they are no longer allowed then we advice removed them from your game as fast as possible and to always abide to the Terms of Service and EULA you agreed to when making your account.

Vendor Price https://www.curseforge.com/wow/addons/vendor-price

This simple addon is useful for those looking to collect and sell loot to earn some gold. It shows the price of an item even when you're not at a vendor so now you don't need to run back just to see what the price of it is. You can collect things knowing it will sell for a decent amount or collect things knowing if you will put it in the Auction House or go to a vendor instead.

Auctioneer https://www.curseforge.com/wow/addons/auctioneer

The Auctioneer Suite is one of the most downloaded addons around for a reason. It gives you the tools and date you need to make auction decisions with more confidence in all areas of the Auction House including when it comes to posting or buying. It gives in-depth information for all game items including those used in quests, enchanting and more.

Questie Classic https://www.wowinterface.com/downloads/info24994-Questie.html

While this one isn't focused on helping you sell items for gold, it does help you with quests which are also useful for collecting more gold. Questie adds icons to the map for the quests in your log and also shows what quests you can get which is perfect for those looking to use quests as a way to get more WoW gold.

Deadly Boss Mods – Classic https://www.wowinterface.com/downloads/info24921-DeadlyBossMods-Classic.html

This is a must-have addon for World of Warcraft Classic who anyone who engages in dungeons or raid content. It adds event warnings, timers for the next events and also gives some advice. The mod comes with a range of voice packs also. For people looking to farm dungeons, this addon will be crucial as a solo player and also for groups looking to earn as much gold as possible.