Not necessarily if you own the home. If there's a room with a wall against a bathroom or laundry room, you may be able to easily tap into the supply and drain. Open a chunk of wall, shut the water off, tap into the supply (depending on what's in there, PEX, copper, old houses may even have steel). Copper or PEX is pretty simple. Then PEX to your sink. Tapping into a tub or shower drain might be tougher, but remember you're doing a counter-level drain so maybe not bad, and the existing drain is most likely PVC unless the home is pretty old (hint - the laundry room wall will already have a drain pipe going up to about 3-4', easy pickin's). If the house has a crawl space, the world is yours as far as layout for plumbing.
It doesn't have to look good, you can patch the wall back up with trim pieces for the plumbing, and you can make a great sink from plywood and porch paint and 2x4 support, stick a shelf under it. My 30x40 developing tray is plywood and porch paint and has held up great, and it probably "holds" more water than a darkroom sink ever will, since sinks are mostly for spill protection and ease of cleanup. You'd probably want a cheap faucet and one of those hand-showers with a hose (or a dish-sink version) would be great to have. When you sell, tear it all out and patch the wall back up.
If you're not a DIY kinda guy, you can probably find a handyman or plumber to do the work - it's really not complex, shouldn't require a licensed plumber since it's essentially a "temporary" thing. Really, in such a situation, my #1 concern would be what's on the floor and under it - carpet on subfloor, I'd pull the carpet up and slap some linoleum down. at least where the wet side is.
If you want a darkroom bad enough, I could see the whole thing costing less than two or three hundred bucks or so, even less if you can do the work or have a buddy/cousin.