All of 1.8 trading has a problem. I don't mind the idea of using other trades to unlock all previous trades, but it has a major flaw: the villager can be locked out permanently. To fix this I propose that on the last trade (the only one not locked out yet), no matter what it is, when it gets the red x it gets a 100% chance to unlock them all again.

The main villager I have a problem with is the tool smith: 6 trades if a pure 1.8 villager. I am very careful with villager trading: I use up one trade until I can't anymore, then only trade other trades one at a time to increase the chance of the first reunlocking. That would mean for a tool smith I have 15 - 60 tries to completely reunlock the villager.

One other thing is that when you trade with a villager just once it may or may not unlock the previous trades, but it also has a detrimental effect on future tries: A new villager trade (brand new or just locked and recently reunlocked) has 3-12 tries. Trading with the villager once may reunlock trades, but it still counts to that tries number. To reiterate, you could get the villager to a point where it will only have 1 try per trade left. This isn't a problem on sp as much to where you can switch around different trades so this doesn't happen, but on a server it is much harder to let everyone know which trades have been messed with yet.