These are starting points but most used tractor equipment is LOCAL. A rotary cutter that sells for $1,000 in Utah might not be found in Maine.
I've spent a LOT of time stretching my tractor $$$. I use Everything Attachments for reference new prices, and BigIron and Tractorhouse for used references. However since 2009 I peruse CL daily and have developed a "mental data" bank that tells me when something is a good price versus a great price. So when I saw a KKII 60 tiller in good condition for $800 I new it was worth it.
I read many posts on here writing "well you can pick on up on CL for $XXX easily". That's not necessarily true. I looked for a plow for months, didn't see one that wasn't rusted thru. Looked for a PTO
chipper, couldn't find a used one for less than the cost of my Jinma. WASN'T looking for a chainsaw and found my Stihl 088 with Alaskan Mark II chainsaw mill for $450 w/ bars, chain, cases, barely used and just tuned at a saw shop.
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Well worth $1000.
One of the things that needs to be emphasized - terms of condition - "good" often means not falling apart, "slight rust" means holes smaller than your fist. I hate driving my dually 50 miles to save $200, and find out the pictures were VERY carefully taken and the equipment wasn't worth the drive.