Protect Privacy When Selling Your Old Phone

I am aware that newer phones have good protective measures when it comes to erasing your data, but what about the older models? I had an old LG VX6100 lying around and decided to put it up on eBay. I did the usual master reset and erased anything I thought might expose any personal information. But is there still a privacy risk, or is selling your phone just generally a bad idea? Recently I heard that people can still access all your personal account data, so I would like to be sure before I actually give it away. Thanks for any help.

Here is my hardware:
tinyurl dot com / 6dbj9q

If you did a master reset it would be nearly impossible for someone to get old data off the phone.