I just did a quick NADA price check on a 2014 Springdale 282BHSE using a Nashville zip code and got these numbers:
Base Price $25,965 Low Retail $12,100 Average Retail $14,600
Given the 25% markup on MSRP, most probably sold for around $19,500 plus tax. I'd suspect around a 15% depreciation the first year and 5% each additional year. Doing that math, I get a price around $14,950.
That's using the purchase price of 75% of MSRP @ $19473 then 15% the first year would be $16552 and then 5% for 2016 would be $15,725 and another 5% for 2017 would be $14,938... I rounded up to $14,950 which would probably be close to what you'd get.
Your price of "around $20,000" is probably more than it originally sold for new. That's assuming your friend actually negotiated a price rather than paid what the dealer first quoted.....
ADDED: I just did a quick search for 2018 Springdale 282BH trailers for sale and got these results:
Advertised price / Link
As you can see, brand new Springdale 282BH models are selling for around 22-24 thousand. Not many people would buy a 4 year old used model for an asking price of 20K.
As a used car salesman once told me, "There's a fool for every seat" so you may find someone to buy it for as much as a new one, but not if you're looking for a "fast, clean sale"..... Just my opinion