Really! I have two R2R DACs that either are 20 years old or uses DAC chips that are from the era of highly praised chips from Burr Brown and Phillips that sound "pristine" with no change in their performance at all. So, your sales person is giving you a bunch of BS, in my opinion. Question, does he only sell Sigma Delta single bit DACs, if that's so that might explain is unfounded assumption regrading R2R based DACs.
Downside to R2R Ladder DACs?
A sales person I generally trust told me to steer clear of used R2R DACs, since their reliance on high precision resistors causes them to sound best when new, and degrade fairly quickly. It seems reasonable; have others had any experience with this?