Have you checked your local second-hand stores or listings?
How basic are we talking? I assume you want velocity sensitivity (louder if pressed faster?)
Post your piano/keys stuff here. Ripping some Chopin, Beethoven, Bartok, or whoever? Love it. Just got a new Hammond and rotary speaker and want to show it off? Can't get enough of it. Got a band with a really awesome keyboardist, and want to show them off? Gimme it. Stuck in the orchestra pit with nothing to do for three hours waiting for your one moment of celesta glory on The Magic Flute? I feel your pain. Just discovered Oscar Peterson and want to share your astonishment? Fam, same. Learning piano and got noob questions? I'm here for it. Got an original DX7 and want to show us all how to program new voices? I've actually never figured that out, can you teach me too?
Rules:
Have you checked your local second-hand stores or listings?
How basic are we talking? I assume you want velocity sensitivity (louder if pressed faster?)
Real basic. Velocity sensitivity would be a nice-to-have. I just need a reasonable selection of voices, midi connection in case I want to plug it into the computer, maybe a pedal.
I got my current keyboard from the thrift shop. I'll probably go to Facebook Marketplace first, but still I'd need to know which brands and models to look for, and which ones to avoid.
ETA: compactness is really my primary concern. My current keyboard has more space dedicated to buttons than to keys. My space is very limited, 36-37" is a hard limit, and smaller in every dimension is preferable.
If you only want keys I'm sure there's good compact options. I don't have any specific recommendations as I mainly use midi keys and only collect old toy keyboards. If you're willing to get produce the sounds from your pc/phone/tablet then compact and cheap options should be plentiful, but that's not as convenient.
Either way be on the look out for a good deal on a Casiotone.
Yeah I'm leaning toward the Casiotone CT-S300
I'm only really familiar with the older models but those are getting more pricey now that more people are on the lookout for them but you can score good deals. For 61 keys that appears pretty compact. I forgot to ask about your preferred key size.
Normal-sized individual keys, 61 keys seems like the sweet spot for number.
If you have the space that's an excellent number of keys. I have made do with much fewer.