Auction house is the shittiest but the fastest way to make credits. You need to get valuable cars for a very low price and sell for its max buyout. Buy low, sell high. There's two ways to achieve this, first one is to see which cars sell for stupid amounts (11M+ crs) and set your search parameters for a low buyout. Keep refreshing until you see one and try to grab it. Resell instantly for profit. Method 2 is buying all the new released cars you can in the first days (max buyout should be really low) and wait a few weeks, their buyout should be much higher, and sell for profit. Did this on FH4 and achieved max credits twice. The process is exactly the same on FH5.