My .02. On modified rigs, there is no real gauge or scale to value them by. Yes, things like cost and type of mods, workmanship will play a part. But, the market is fickle on them. It all depends on the right timing. One month that could easily go for $65K because it was exactly what that person wanted and was willing to pay. Partially modified are the worst. They have lost their collector value and don't have enough mods to be worth a big price. This one is that category. The flip side is, if that one was properly disassembled, repaired, and refinished top and bottom, it would cost you at least that to pay a shop to do it, if you're very lucky. That one clearly had the wrong write up in the ad, which also really said nothing about the rig.
Now, about OEM, that is usually down to condition, so easier to scale a value and also, you know what is in it.
That shop should list their 40 on BaT. Most weeks, those folks will buy anything. But, it's all about timing. List it at the right time and there will be a buyer all over that for that price. List at the wrong time and the bid will stall at $25k and won't meet reserve.