As Tony Bose has said many times, making slip joints requires a real bad case of the 'want to's'. Like most knifemaking, if you're in it for the money, you'd probably be better off doing something else. Personally, I love working on slip joints, so it isn't about the money for me. I get much more satisfaction from working on something that is a little more challenging and would be something that my grandfather would have carried. If it were easy, everyone would be making them.
By no means am I an accomplished slip joint maker, but I am trying it as well. I think the single biggest mistake that us newbies make is trying to make slip joints from a clean-sheet design. The people who buy and collect slip joints typically embrace the tradition behind them. As such, they prefer traditional patterns versus something drawn up from scratch. My first slip joint was something I drew up entirely on my own and it turned out horribly. Since then, I've been working off of ESTABLISHED traditional patterns with much better success. My inspiration came from Kerry Hampton, apprentice so-to-speak to Tony Bose, who recommended that I spend $17 to buy a Case Sodbuster Jr. at Lowes, pull it apart and use as a pattern. I did just that and my second slip joint turned out pretty decent. I've since gotten some patterns and are now working on those. Unfortunately I'm really, really slow, but am making decent strides in fit and finish on each of them.
Continuing with my recommendation above, you also need to put forth the effort to see some truly old knives. Examine them and learn what distinguishes old Sheffield and early American pocketknives from those produced today. Take note of the fit and finish, swedges and rounding of the handles. It seems to me that most slip joint users and collectors want knives that approximate those of old.
Lastly, work on your nail nicks. Compare yours to those of accomplished makers whose knives sell for $250+. When I first started, I put some money into a 6" grinding stone, had it cut to a 45° with a knife edge. My plan was to grind in my nail nicks after heat treat. I've since found that many collectors/users prefer nail nicks that are cut (i.e. machined) in prior to heat treat. Nicks cut in this fashion are almost always crisper than those ground in after heat treat. I was surprised to learn that many prefer the nick to be discolored from HT rather than a shiny finish as achieved by grinding post HT. For ~$35, you can buy a HSS 1 3/8" 45° dovetail cutter from Enco (assuming you have access to a mill) that will enable you to do this.
If you have any other questions, please do not hesitate to ask. Also, please take all of this as constructive criticism, not as me attempting to bash you or your knives as that certainly is not my intent.