If it is an intermittent fault, you might be sending it back and forth dozens of times before the vendor actually sees the fault. Especially if you are paying for the postage.
If you are keen to just sell it, and only one LED has an intermittent fault, you might try selling it as-is with disclosure, and maybe someone who is comfortable with soldering would buy it and fix it.
Or, replace the LED yourself. It is probably not worthwhile to pay someone to fix it for you.