They explain what they are about here.
My thoughts on the matter.
First off One Bookshelf or OBS is the leading on-line seller of on-line RPG products both digital and print on demand. They run the RPGNow and DrivethruRPG websites They also offer cards, cardstock, and small poster printer to allow products like paper models, and card games to be sold as well. OBS is the 900-lb gorilla they are competing with. Since they are focusing on OGL products and the OSR, you have to keep Lulu in mind as well as that is a major source of many OSR publishers sales.
Tabletop Library is offering a 75% non-exclusive royalty rate for digital PDFs compared to OBS' 65%. For physical products they are offering a 65% royalty rate. Unfortunately they are not clear on what that entails but looking at the FAQ I would have to say it appears they are offering to warehouse and ship any physical product you ship to them.
Tabletop Library appears not to have a print on demand service yet. However they are talking about kickstarter fulfillment although there are no on-line details yet.
The website looks great and is very clean and easy to navigate I view this as a good start similar to what attracted me to Google over Yahoo. I also setup an account and that was pretty easy. The publishing interface looks pretty clean and easy to you. They are explicit about current bugs and limitation and clearly state what you have to do now to properly post a product. I didn't have to go fishing for instructions. Definitely get a A+ in terms of how their website functions.
I do think they are making a bigger deal of competing with the DM Guild than they ought too. RPGNow/Drivethru supports traditional 5e OGL as well as stuff published through the DM Guild. What the DM's Guild gets for the 50% royality is to play with Wizard's IP. If your project doesn't rely on WoTC IP then don't publish through the DM Guild, just publish as a normal publisher on RPGNow. Personally I am not interested in publishing for Forgotten Realms so the DM's Guild holds little interest for me. However if they add Greyhawk to the list of allowed IP then yeah I have some ideas that I would publish for the DM's Guild.
I think in the long run Tabletop Library is going to need to offer print on demand. Hopefully their current setup will get the volume needed to get one of the various PoD providers to give them a decent deal.
My conclusion is that OBS can use the competition. With a focus on warehousing and kickstarter fulfillment they have a shot at carving out a niche as a foundation to take on OBS as long as they expand to offer OBS' range of services. Tabletop Library web site looks good so far and if they successfully maintain the minimalist interface that will help them a lot.