Just tried this myself last week and have been playing regularly online and on the storefront etc with no issues
All i did was
1. Created another account on my xbox
2. Signed up for the free month trial on xbox live gold with THAT account, (note you will need to create another email account or use a secondary that you have)
3. Once created, sign into your MAIN account and fire on Forza 4
4. Find or purchase a car, cheap, and put up for auction for the maximum, 999,999,000
5. Now head to home and sign into your newly created Free trial account and fire up Forza 4
6. Do the first introductory race and pick a starter car etc, then make sure the game autosaves, or change the paint to make it save etc.
7. Once saved, get that save off your console and onto your computer, then mod your credits ( with one of the many editors now available of course).
8. Save Rehash Resign etc and now back to the game on your FREE account
9. 99% of you will now have guessed this part but anyhow, with your shiny new, yet rich, profile, head to the auction house and locate your overly priced auction put up by your MAIN account
10. BUY :) Job done, and turn 10 cannot touch your main account for you didnt do anything illegitimate.
You sold, SOMEONE purchased, whoever that nutter is lol
Recommend only doing this once and once completed, possibly deleting the FREE account
It's merely a quick 999 mil and not really intended for constant abuse, as of course you may indeed get console banned
Thanks odd future for setting me straight there :p