And there is obviously no difference between finding a population centre with the listed GP ranges and finding one that will buy all an adventurors parties crap, at market value, with no warning and then be able to supply any item of that GP value or below, with no warning on the purchase. Oh, and any partially charged version of those items as well... Because that's far more likely than having downtime in a campaign... What???
What you're arguing isn't that you can find a population centre, it's that every population centre is as flexible as Sigil. I know the DMG says they'll have a certain amount of cash available but assuming any and all frounter wholesale farming+fishing store with a sideline in supplying standard weapons and armour to have a safe in back with Everything you could want, as charged as you want is a bit dubious. Buy all the hoes, seed and horseblankets you want, shifting that shiney doohicky's gonna take some time.