As you have found, it's not really possible to find a definitive price. There are so many variables. Location, condition, season, etc.
And then you have seller variation. I tend to price on the low end, because I don't want to have to "sell," the trailer to the potential new owner, nor do I want to have a trailer for sale
for any longer than necessary. I would rather price a tad low, and then have a quick sale and a happy buyer. But that's just me.
At the other end of the spectrum is the sort of person who will sell for a certain price only, and they don't mind if a buyer doesn't come along until the cows come home -- or never. They have their price and that's that.
Then there is everything in between.
With upgrades, they can be a feature or a detriment, depending on how they were carried out, and what the buyer likes. Original condition is nearly always a safe bet (which is not to say that one should not modify their trailer, or that modifications cannot be improvements).
I would read through the archives of "sold" and "referrals," keeping in mind that those are asking prices. You can also search "completed listings" on eBay. It's all still a process with no hard and fast rules though.