https://www.bicyclebluebook.com/

That's a place to start...

Ask whatever you want.

However, be prepared to sell it for around $1,300.

Depends what bike. Any upgrades? How quickly do you want to sell it? Bicycle Blue Book often gives a value on the lower end, so I'd take it with a grain of salt.

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Strava

I was told that a pretty good rule of thumb is that a bike has a depreciation of about anywhere between 15% and 30% a year. (1st year 2500-30% = 1750. 2nd year 1750-30% = 1225. ) So the value of your bike would be anything between 1225 and 1800. Mileage is something you can never prove on a bike. Also take into account supply and demand, where i am there are not that many decent used tri bikes for sale which drives up the prices significantly. If it's a bike that is still in demand you could easily get a lot more.
Make sure it's all shiny and really clean when you sell it. Take loads of good pictures if your selling it online.

Could depend on factors (manufacturer and size come to mind), but EBay and ST are a good place to start for guidance. But, ballpark, I would probably expect that bike to sell between $1,000-$1,500 (keep it mind you can probably now find that exact bike, new, with a warranty, on sale for under $2,000)

^^^This.

For reference, I bought my roadie (a 2016 that retailed for $3200) for $1750 end of season 2017 (it had been owned/raced ~18mos.).
The mfg was selling the leftover 2016s direct for $2500. Mine even had upgrades that the original owner had added ($175 carbon Fizik saddle, $270 Enve seatpost, and a DA cassette).

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Don't ask me where I got the number from, but I usually buy or sell clean used bikes for about 60% of what you could get a new bike for (retail and maybe minus a bit because you can get maybe 5-10% off by haggling).

The disc brake revolution could really hammer the value of your bike. That is sort of the future so there probably won't be a lot of people willing to pay decent prices for a rim brake bike.

Prices above seem about right. Whether it sells easily or not also depends quite a bit on the credibility of the seller. I turned up to look at a "pristine, low mileage" MTB on eBay once, it was superficially OK but when I looked a bit closer I spotted that the braking tracks were totally worn out, as were the teeth not just on the cassette but on the chain rings as well. Based on my experience I reckoned that meant it had done something like at least 10,000+ miles of regular, well maintained riding or 5,000+ miles of being ridden in harsh conditions or with poor maintenance. I've also had a lot of friends who know less about bikes than me and have been burnt by buying supposed pristine second hand bikes from eBay and then finding within the first year that pretty much every part that can wear out has worn out. Or in one case that the bike was stolen.

So unless I'm buying from a trusted source with some kind of warranty, I would only buy a secondhand bike if I can view it first before committing. I would also much rather deal with somebody who I can at least verify is a somewhat serious cyclist/triathlete who knows what they're doing e.g. they're a known rider from my local community, are vouched for by somebody I trust, or at least I have their real name and can do a bit of Strava sleuthing or something. If it seems the seller is genuine but a bit clueless, e.g. somebody who bought a bike wanting to get into cycling, barely used it, and now wants to sell it, then it might be worth a look as long as they're not too attached to the idea of getting most of their money back.

Also worth pointing out that if you're in the UK (unlikely given the use of $!) then the value of secondhand bikes is considerably lower as there are tax breaks on buying a new bike, and they've recently scrapped the cap on what you can buy. As far as I can tell, you can buy basically any bike you want out of your gross income with no income tax (which is 40% for a lot of people) and no VAT (which is 20%), so effectively about half price. Used to be capped at £1000 which meant you were limited to more entry level road bikes or commuters, but seems you could now go out and buy yourself a £5k bike for a net cost to your wallet of ~£2500. So I can't really see why anybody with a job would buy a secondhand bike unless it was discounted to well below half price. Which also gets me thinking that there must be a business opportunity there to start buying up secondhand bikes in the UK at depressed prices and then export them to countries that don't have a similar tax break...