I bought an old LeMond Buenos Aires in mint condition a few years ago that was a replacement for an Alp d'Huez I rode for 20 years then sold. The BA came with 10 spd Sram Apex shifters and deraiileurs. I've put about 1k miles on it and I just don't like the Sram doubletap shift action, so I bought some lightly used Tiagra 10 speed shifters to put on. So now I need some derailleurs and my question is: should I go with a used 105 10 speed derailleur or just get a new Tiagra one? I'm worried a used derailleur might already have significant mileage and wear on it. I am quite familiar with what a worn derailleur is like since I replaced a Shimano 600 that was completely shot, last year.

if the Tiagra is 4700 then you need either 11 speed or 4700 rear derailleur. If it is 4600 then you can use any ten speed road derailleur. I personally really wouldn't use derailleurs or shifters from Tiagra but if I really needed a budget bike 4700 isn't so bad. 105 is generally my lowest on the road side.

If you are unsure of which version Tiagra shifter you have, look at how the shift cable is routed. The ST-4700 has the shift cable routed under the handlebar tape like the brake cable. The ST-4600 is the older type with the exposed cable exiting from the shifter.

As already noted, each version needs different derailleurs. The Sram front derailleur may work with the ST-4600, (trim position may be a little off). The ST-4700 uses a newer style FD.

Here is a chart to find the "pull ratio" of many derailleurs that has to match with the shifter ratio which is what has been mentioned above and why the 4700 version will not mix with the 4500-4600 and some other older Tiagra groups. You need to know which model shifter you have before choosing a derailleur. Two Wheel Tools NOTE: This is the same chart that gets posted here occasionally but has been moved to a different host with a slightly different format but has the same info. (I liked the old format better)

The Tiagra brifters have the model number on the outside of the body just like every other Shimano brifter. No need to guess.

There is nothing wrong with Tiagra. If the levers are 4600, any Shimano 8,9,10 speed derailleur will work, excepting 4700 components, and I believe specific DA level. I have used 4600 levers for years, matched with 6600 components, always solid performance. The big difference, is the 4600 has exposed housing. They work better than the 5700 I have, both smoothness of shifting and consistency. The first generation of Shimano STI levers, at least the 5700-6700, had issues with having to do a lot of tweaking for the shifting.