You can sue your cousin for the remaining $225 in small claims court (filing fee in most jurisdictions is around 35-50 dollars). You could attempt to sue in the regular division (higher filing fee) if you want to try to get the court to order the return of the Wii, but you're unlikely to prove your case that the agreement contained a purchase money order security interest (PMSI) wherein you could repossess the Wii for non-payment if there's nothing in writing.
You may also have difficult proving that the purchase price is $350 and not the $125 she paid unless you have something in writing by her confirming the purchase price. However, given that new Wii's generally cost over $300 on the market, a judge very well may likely to find that your price is credible.
Small claims court is a process that requires no attorneys and is very informal. Matters are heard relatively quickly as well and it's inexpensive. However, small claims courts can only issue a money judgment, it cannot order the return of personal property. Given the costs of litigation, (un)likelihood that you could prove to the court that the deal also contained the right to repossession, the most cost-efficient way to resolve your dispute is for you to file a small claims complaint at your neighborhood's municipal court (county court, not common pleas, if your area doesn't have a municipal court.)