When is a

I'm assuming that the performance of most cartridges starts to go downhill after a certain amount of play. Is there a rule of thumb for when a cartridge needs to be replaced (for example, after 4000 hours)?

Are "used" cartridges a good buy and, if so, are there any general guidelines for buying "used" cartridges?

Can all cartridges be retipped?

4,000-6,000 hours wouldn't be a bad rule of thumb, but, regrettably, the "rules of thumb" just don't work out very well. Too many variables and differences among the cartridges.

Buying used catridges is a major gamble, imo, unless you know and trust your seller. I suppose there are people who successfully buy used cartridges, I wouldn't chose to go that route myself. I'll be interested to see what responses you get from folks who've done this.

Re-tipping is not the entire issue. The cantilever suspension material degrades over time. Unless the cartridge can be rebuilt, retipping won't address degradation of the elastomer material used in the suspension.