You can easily get both Action House achievements with a second account (doesn’t need to have gold but if the Xbox you’re using isn’t your home Xbox, keep your main account signed in while using the dummy account). You’ll have 100k credits when first starting the game.
Do this with both accounts.
Navigate to Cars and select My Garage. Select the 2019 Hyundai Veloster Turbo (any car both accounts have will work). Choose to Auction the car. Choose the lowest buyout price on the dummy account. Feel free to use the highest on your main. Confirm the auction.
Now back out of the garage and select the Auction House. Search for the car (just change Model to Hyundai). Find the car and choose to Buy Out (shows the seller below the car). Select Collect Car and a few seconds later the Buyer’s Market achievement will pop.
Now with your main, return to the Action House and navigate to My Auctions. Select your sold auction and choose Collect Credits. A few seconds later the Wheelin’ Dealin’ achievement will pop.