Encryption is a good step. But, it's really not a problem to securely erase your HDD after you are done with it. Securely erasing involves writing a varying sequence of bytes over all portions of drive. The objective is to make it impossible to extract data from the magnetic platter even though it has been overwritten by some other data.

If you merely delete the file, the data is still there but the pointer to it in the directory has been removed. This is completely unsafe.

Overwriting the file with some other data is not enough either. This is because of the magnetic properties of the HDD platter. With the right equipment, you can actually recover bits that have been overwritten with other bits.

A secure erase will take care of it: Overwriting the bytes a sufficient number of times with varying byte values makes it impossible to recover the original data.