The video post below shows how to do this yourself. It's very easy and takes about 10-15 minutes to do one deck. I use the jumbo size Avery Marks-A-Lot Permanent Marker but you can use the medium size. As long as it has the chisel tip you're safe from making mistakes. This wide chisel tip prevents accidental clumsy marks that can occur when using a non-chiseled tip. Since it is perpendicular to the card, it's rather fool proof. Watch the video, you'll see what I mean. I only show one card as an example to keep the video short but I think you'll get the idea. It's important that you ink one card at a time. It doesn't take that long and it will eliminate any bleed through that may happen from trying to ink all the cards' edges in a stack.
The reason I use The Avery Marks-A-Lot Permanent Marker is because it is non-toxic, low odor and non-fading. Other brands smell and fade over time. I have no affiliation with this company. I have tested a few markers out and this marker particularly is the best, IMO.
When you're finished, you'll have a lovely black faux-gilded edge treatment. It can become addicting, trust me. I've edge-treated several decks in my collection. :) I also use Tim Holtz Ink pads for some of my decks. You have to be careful with his inkpads as they are water-based and could be too wet for your more precious decks. His inkpads fade easily since they are not permanent. I have also used his markers, which are also water-based and act the same way as his inkpads. So please choose your decks carefully when using the water based or the permanent inks for that matter. I would suggest that you don't use your delicate or treasured cards!
*It's best if the borders on the front and back match.
*Try any of these tools on a test or title card first if your deck came with one.