If you decide to go with the one that has not yet been registered, be sure to get the original receipt from the seller. That will be the key to registering your DF500.
When I got my TS55F, I bought it used from a seller who had bought it a month before. At the time, he was asking $600 with the 1400 FS/2 rail and we agreed to $550. He had bought it online, had not registered it, and sent me the electronic receipt.
When it came to registering the tool, it was just as though I had bought it new. Went to the website, filled out all the information and included the jpg of the receipt and voila - registered.
Based on comments about the DF500 from other threads over the past year, it seems that there have been very few mechanical issues with the DF500, so buying used (and outside the warranty) should not be terribly concerning. Of course, you should scrutinize the tool, both in pictures and in-person, before finalizing the deal. If you've watched the used Festool market, you'll know it's not uncommon for sellers to be asking a considerable amount of money for a tool they state "used once", "used on one project" - and the tool looks like it's been through hell, or is a decade+ old (this can be quickly ascertained by the Systainer the tool is in and the date on the manufacturers plate).
Speaking of Systainers, also consider what the DF500 comes with. Does it include all the accessories of the current version? If not, how much will it cost you extra to bring it up to speed? And are those accessories important to you - and at this moment to get working?
When Festool updated the OF1010 earlier this year, the old SKU went on discount and a dealer was selling the old SKU for $500 - sounds like a great deal. But when I calculated what I would need to buy to bring the accessories up to the current assortment of accessories, the savings were a wash - and if I bought the accessory kit, i would end up with two rail adapters.
Currently, there's an OF2200 on Marketplace with the seller asking $550 - which is 60% off the current MSRP of $1220. But it comes with nothing. Getting the spanner, collet, adapter, dust collector, M337 Systainer and insert will cost about $300, bringing total cost to $850 - equivalent to a 30% discount off MSRP - but for a tool made in 2012 with no warranty. That said, it's still the lowest price I've seen on a used OF2200, but only by $50. Worth it? Maybe. The OF2200 is a beast that has no equal and I don't think I've seen anyone reporting issues with the tool.
But 15% off? If it comes with the receipt then all should be Golden. If not, I'd rather see a steeper discount or just buy it new with the full warranty. Also, another thing to consider in your calculus is just how long ago the tool was purchased new - if a month: yes. If two years and six months? That's a little different.