With the caveat that I'm not a savvy eBay shopper, I'll offer my opinion, cobbled together from my previous replies on the subject.
Resale on high-end espresso equipment is good. Mazzer grinders typically go for 50% to 70% of the current retail price. Resale value of used espresso machine is less reliable since their value is tightly coupled to popularity. If you pick a popular model, can sell for the same 50% to 70% of current retail price if kept in top condition, should you decide to upgrade someday.
If a used model still in production with average popularity were in pristine condition but recent models have seen improvements in feature/reliability in the interim, the best price I would hope for is around 50% of today's model's full retail price. The price falls precipitously if the question of needed repairs enters into the equation, i.e., a few hundred at best.
With apologies for my laziness, other related tidbits cobbled together from my previous replies:
- Unlike many hobbies (golf, audio/video), you reach the "high end" very quickly. Above $1400 at full retail, the differences among espresso machines is vanishingly small.
- The same holds true for grinders. They peak around $700 (flat burr) to $1400 (conical) at full retail. You could spend over $2000 for a Mazzer Robur, but unless you're running a cafe, there's no rational reason to consider it.
- There are few purchases that will give you and your family/friends pleasure every day. That's why for me, when it comes to espresso gear, I shop on capability/convenience/quality first and price last.
That said, there's lots of fools overpaying on eBay everyday, so take the above comments on resale value with a grain of salt.