Depends what kind of diving you're into, cold and deep or warm and shallow.

What certification and experience you have.

I've dived under 2m thick ice at 20m in Norway, with great visibility, to 0m visibility and 105m depth in the North Sea, to boiling hot (22C for me anyway) in the Indian Ocean and crystal clear water. The set-up for each was pretty different. You adapt your gear to suit your needs.

The regs I needed for Ice diving had to be pretty robust as to not freeze. The regs I had for deep diving suited the mix of gas and tanks I'd be using to make my descent and ascent. At that depth it takes hours to come safely back to the surface, you're in the water along long time and need to be equipped for it.

For mask, fins, snorkels, wet suits and dry suits go with ebay. For regs and stab jackets go new buy from a shop you know. You can't always be sure how well the regs or jackets have been maintained when you buy from ebay, unless they're new from ebay, but then again are you sure the jacket will fit well?