I’d suggest buying an old retail version of PhotoShop. You can create Action in Ps to automate the process such as auto saving batches of images you have cropped.
I first open all photos in batches in Mac Preview and do a quick rotation of any images into a upright position that require it. I also delete any images that are not useable. Way quicker to this using Preview than PS.
Images are dragged into the Mac app Name Changer and bulk renamed in the order that they should appear in the listing such as piggybank1.jpg, piggybank2.jpg and so on.
Images are then dragged into PhotoShop in batches and cropped. A few may require a slight additional manual rotation of the crop tool before cropping.
Some images may require a color correction, such as very light objects on a dark background. For individual images I just press Command + Shift + B. For batches of such images I use a custom Action which color corrects all such images currently open.
I then use a Action to auto save all open images so you need not do this manually for each one.
Images are then batch watermarked using PhotoMill - a watermark copy of the image is saved for insertion into the listing.
Recently I created a app using Xojo which allows me to batch merge multiple photos into collages.
My camera takes 5184 x 3456 images. The app will combine six of these images into one image using a 3 x 2 grid (155552 x 6912 pixels) in less than 2-3 seconds. eBay will further reduce the size of the image on their servers to a maximum of 1600 x 1200.
This allows; for instance to combine 48 images into 12 images using twelve 4 x 4 grids.