I would like to put a border (top and bottom) around a bunch of images to get them ready for item listings. Is there a way to go about doing this? Sorry for my ignorance, but im new to Gimp

On the face of it this looks simple but there are snags Two cases

1. The 'y' border top and bottom is equal. Filter -> decor -> Add border will do this providing you accept a 1 pix border (minimum value) on the 'x' sides.

2. The top and bottom border differ in size. The usual way in Gimp is Image -> Canvas Size with new total height and an defined offset.

For a batch process there is a Gimp plugin BIMP see:

https://alessandrofrancesconi.it/projects/bimp/

As well as built in functions, resize etc it can use the regular gimp procedures as 'other gimp procedures'

The first option 'Add Border is relatively straight forward. Apply add border, apply flatten image, apply save as. All in BIMP.

No 2 - different sizes is a bit more difficult as BIMP does not recognise the Gimp resize canvas procedure.

A bit of 'outside-the-box' required. The Gimp script apply drop shadow can be used twice as a solid colour with suitable offsets in BIMP.

BIMP can save actions in a .bimp file, Attached an example.

Install BIMP, open that file, add the images, give it a try.