I cannot help you with a value, but plenty here can, so that should not be a problem, but you may need to tell them when you bought it!
But I can assist in keeping the value up just by keeping the battery working, as they simply do not like being ignored for long periods. A long period could be as little as 3 months with some.....But quality batteries last much longer.
So regular charges, (not necessarily a full charge, just one that gets it to the middle of the voltage range between fully charged and discharged. For a 36 volt battery for example, that would be around 35-36 volts, say once a month, and usage, so go for at least one long ride per month. (Or get someone else to do it!)
A voltmeter is needed, and the knowledge as to how to safely use it.....even a cheapo meter, around 10 UK pounds, should be accurate enough. Compaing it to a well known good meter will help, and the differences on the range you need, should not exceed a volt at worst.
Nothing reduces the value of an e-bike more drastically than it having a defective battery......But so easily done...... Out of sight, out of mind.....
When selling and buying most vehicles the value is generally well understood and a fair price easily worked out, but with e-bikes there's clearly a problem since prices asked are often unrealistic and in many cases owners and buyers haven't any idea what the right price should be, as shown by...