Are used shocks a bad idea?

[Grimly]

Yep, lots of barely used Harley shocks around, and some are 13" eye-to-eye. No, they're not great quality, but likely better than 40 year old worn-out originals.

From what I can tell, most have 1/2" eyes. That works out to about 12.7mm which is slightly big but close enough to 12mm to work OK.

Simple to fabricate a bushing sleeve out of a tin can.

[LAB3]

Hit a swap meet, as mentioned HD guys swap them out often because it costs money and makes their bikes better because they cost more.

[Seth]

I did the $20 shipped used shock dance and it was a waste of money. The photos were not good enough to see how roached the bump stops were and they didn't have an ounce of rebound to their name.
However, I found some take off shocks from a '17 Triumph Speed Twin fit up nicely once I got some Mcmaster-Carr sleeves to size my studs up. Apparently it's pretty popular for guys to upgrade their Triumph shocks with Ohlins stuff that Triumph sells in their aftermarket store. This leads to lots of low mileage spares on ebay. I picked mine up for $45 shipped and paid maybe 12 bucks for the sleeves? A much nicer ride, and they look like O.E.