I am looking to apply for a UK YMS visa. I previously held a UK study visa in which I overstayed for 8 days but left voluntarily. Since my overstay I have also returned to the UK for a short period on a visitor visa in which I had no problems at the border. Is it likely that my current visa application will be rejected due to my overstay previously?
The fact that you have been since your overstay on a visitor visa does help the case, you will need to show the immigration officer that you will return to your home country.
A return ticket before the expiration of your visa will help your case, then the officer will see you are planning to leave the country on that date. Also sufficient proof-of-ties to your home country will help (family, work, etc.). But ultimately, it is up to the immigration officer and no-one can give a definite answer here.