The items you are selling should have some type of certification or grade so customers know that they are the real deal. You will want to have a professional storefront, which you can easily put together using the Selz store builder. Reviews are also very important in this line of business, so don’t hesitate to ask previous customers to drop you a line with their thoughts.
Do you enjoy fast food or going into retail shops? Many companies hire mystery shoppers to test the customer service of their stores. A common one is Subway, which hires mystery shoppers to ensure that "Sandwich Artists" are following all the franchise guidelines. If you have time and can travel to different places, this could be a good side hustle.
As for other tips, all I can say is “a penny saved is a penny earned”. If you are running out of ideas to make money, start saving. Some quick “low hanging fruit” tips to save some quick cash include – switch to MetroPCS or Boost for cellphone (I only pay $20/month), switch auto insurance to Insurance Panda ($25/month), and start using GasBuddy (saves me like $100/month at least. I drive a lot).