Why are you concerned with potential resale value? (ironic, I know ... but I am selling mine for a different and much more rational end)
If there is no sonic gain and the device is in excellent condition, the difference must be made up by differences in functionality, the age of the device and presumably also irrationality. Further, if the device is not to the buyer's liking but as advertised, there is likely not the possibility to return it to the seller as is common nowadays when purchasing new units (e.g. Thomann has a 30 day return policy). Secondly, there is also the possibility that the device is not in the condition as advertised.
Apart from being even more accurate and with exception of the Low Dispersion filter and the new remote, the revised DACs function the same as the original DACs. As the increase in accuracy is very likely inaudible, the new filter would have to increase value significantly. Given that the price for the revised DAC is the same as it was for new units of the original DAC, I think that is questionable. Regarding the new remote, as it can be purchased separately, valuing it on its own is easy. However, in this case, it is to be valued against the old remote. I think the new remote at most adds 50€ in value relative to the old one. More likely, it is around 25€, i.e. relative to the price of the unit when new, this is a decrease in value between 0-5% which still leaves 30-35% loss in value to be explained.
Regarding age, given the reliability of RME devices and RME's continued support for many of their old devices, I don't think this is a decisive factor either. Secondly, four years of warranty should keep the expected cost of operation (beyond electricity) around 0 for some time still.
Considering only those two in sum and presuming the device is in excellent condition, I don't think gives reason to a reduction in value of 30-35%. However, some value is likely lost indeed. Given the prices for used and new units I have seen and, I would put that at between 5-10% leaving still a 20-30% loss in value to be explained.
Given the available information about the RME, I think many prospective buyers know enough about the device (certainly on this site) to form reasonable expectations about whether the device will be to their liking. Thus, the lack of the possibility to return the device does not justify the remainder of the 35% loss of value: 20-30%.
There is of course some loss, as the expected loss to the buyer if the device is not to his liking is likely positive compared to buying the unit new from a seller with a return policy. The difference being between shipping cost to seller and loss from sale.
I have not much of an idea how much that would be but I would put at between 2-5% (maybe someone else can chime in) putting the residual loss of value due to other factors at 15-26%.
Since as mentioned above, the buyer cannot fully evaluate the condition of the device prior to purchase, he faces the risk that the real condition is different than advertised. Given the rather slim chance that the device is in better condition than advertised, the prospective buyer's expected value of the devices is (most likely) lower than his valuation of the device in advertised condition. However, the seller can minimise his (corresponding) expected loss by providing the prospective buyer with information that demonstrates (or at least credibly suggests) that the device is in the advertised condition.
If sufficient (i.e. enough and credible) information is provided showing the device being in the advertised condition, the risk faced by the buyer that the device is not in the advertised condition is essentially 0 and thus should have no negative effect on his valuation of the device. Thus, I think the loss in value due to this would be 0%.
However, if the seller does not (including if he cannot) provide information to the prospective buyer sufficient to demonstrate that the device is in the advertised condition, there is some loss to the prospective buyer's expected value relative to the case of sufficient information and hence to the seller's expected income. Thus for a trade to occur the price would have to be lower than the value of the device in the advertised condition.
How much that would be depends on the degree to which the provided information suggests that the device is in the advertised condition but also that is not in a less desirable condition.
Overall, I think maybe 1-5% lower sales price relative to valuation in advertised condition seems reasonable if the information provided is of good quality. Much lower than the residual loss of value after the above factors of 15-26% leaving still a loss of 10-25% to be explained. (If you find this interesting: look up information economics dealing with incompleteness and asymmetry. For a start, take a look at Akerlof's "Lemons model".)
There is still the possibility of scams but I think the probability for it in this market is so low as to be considered irrelevant.
About irrationality there is much to say but also pertaining to the above particularly forming expectations. Anyways, assuming fairly rational expectations and the provision of high quality information (as you claim to intend to do), the sales price of your device should be between 79-90% of its value when new (i.e. 790-900 €) much more than the 65% (i.e. 650€) you consider. In fact, this much lower price, as per the above, suggests that the device is in worse condition than advertised.
Further, applying the same analysis to a new revised DAC and the gain in resale value is at best similar to the expected loss relative to new price for the original DAC, i.e. between 100-210€ after one year. After more years, the relative loss in value compared to the potential sales price one year prior should be similar as many of the effects taken into account for the first year are also present in the second year but most likely nowhere near twice, trice or four times as strong for a two, three or four year unit, respectively. Thus, you would most likely not be able to recover the 35% loss incurred from selling your old unit at 65% of new price from the increase in resale value from purchasing a new unit.
You would gain slightly (and most likely inaudibly) better accuracy, a different filter and a different remote which can be purchased separately at a much lower cost.
So, as per the above, your expected loss is likely between 140-250€ and more likely at the upper end between 190-250€. Is peace of mind (and most likely, only peace of mind) really worth that much to you when you could use the money on something that really makes a difference?
