Just checked Ebay. I searched completed listings Trek 531 .This is the cream of the crop. It is for the last 10 sales, F= frame only maybe with HS,crankset CB-complete bike
$85 =F under $300 as cb
$99 =CB
$405 =f waaay over $300 as cb-maybe $800??
$209 =cb
$52 F under $300 as cb
$177=F over $300 as cb maybe $350??
$590 =CB
$202=CB
$260= CB
$47 =F under $300 as cb
It looks to me like 3 of the 10 would be $300+ bikes,one would be an $800 bike,and one is a $590 bike.The rest are ~$200 cbs-real bargains for 531 cbs.I didn't check the shipping-if the $99 bike had a reasonable shipping($60-$70) it is a heck of a deal.
The 531's are cream of the crop. I like old steel Cr-Mo mtbs for the same reason I would love these-cheap quality-can't beat that. If I was less gimpy,I would stock up on these bikes, but I need big soft tires.These bikes probably sold for $300+ 1980 dollars(maybe $700 current dollars). You can now buy them for maybe $250 on average for the CB. The decent MTB CR-MO of late 80 early 90's are even cheaper-maybe $125 at best for a $500 bike and many are waaaay under $100.
Luck,
Charlie
PS I don't get the SS fixed gear interest, and sure as heck have no interest in taking off braze ons, so I eventually have to use cheezy cable clamps, or my zip ties.