The problem you have is that everybody knows that stratasys printers have locked in stratasys materials, and stratasys really make their filaments stupidly expensive.

These days for $500 you can buy a really nice idex, stick it in a cheap enclosure and use as cheap a filament as you like.

Do stratasys make good printers ? Absolutely !
Are they value for money, materials wise ? Oh hell no.

The only way i can see to sell a used stratasys for anything other than spare parts, would be to somehow get round the materials lock-in and sell it as a fully cracked open source filament printer.

I'm pretty sure someone somewhere has done this and made a youtube video or walkthough on the details.

But without that, most people just aren't going to want to be locked into the stratasys propritary filament eco-system.