Who sells onn audio tape cassette recorder / players online?

Both players should play a tape. In fact the player can be set up so when tape player 1 finishes the tape, player 2 will kick on to play the other tape by itself. Only one is the recorder to dub another tape or record CD to tape.

Possibly a similar model - free online :

https://fccid.io/G3OONA17AA015/User-Manual/UserMan-3313968

ONA17AA015 CD MINI STEREO SYSTEM User Manual UserMan ACE INTERNATIONAL...

Hello, Edward -

I have only been able to find Service Manuals for the Marantz SD-160 dual deck cassette player for sale.

I was unable to find the operating manual for the vintage cassette player / recorder on the Marantz website.

However, if you have not yet already done so, I suggest you contact Marantz Support, either via phone or email, and ask if there is a copy of the operating manual for the SD-160 in the company archives and if they could provide you (sell you) a photocopy or scanned PDF version of the manual. This is the link to the Marantz Support page:
https://marantz.custhelp.com/app/answers/detail/a_id/104/kw/manual/related/1

If that option doesn't work out, Option 2 is to find one or more online sellers (eBay, Craigslist, Amazon, etc.) of the model of cassette player / recorder and see if they mention the operating / owner's manual in the product sales information. If they don't mention a manual, I suggest contacting the seller/s and asking if they have the manual.

If you find an online seller with the manual, ask if he / she would either photocopy the manual for scan it as a PDF document and sell it to you.

Best wishes.

can look up online using make and model or electronics store in your area

You can't. Cassettes are analog and the USB only recognizes MP3. The USB will only record off a normal CD or a CD with MP3s on it. It won't record other formats. The unit only allows to record from CD to cassette or from CD to USB. There are some cassette decks that will convert to MP3. Just do a search for "cassette to MP3 converter"

find a tape player/recorder than has the capability to connect to a computer connect your tape player/recorder to your computer open windows explorer navigate to the tape player open it then select the files you want to copy to your computer right click select copy then navigate to the place where you want them saved or you can select the files hold the muse cursor on them drag them to the place you want to save them and drop them into it
hope this helps

Radio Shack used to sell a splicing block and tape for fixing broken tapes. They may still, and you can order online from other sources.

You need to pull the tape out from the cassette a little way. Overlap the tape sections a little and get them aligned, which is what the splicing block is for. Cut across both pieces and take away the scraps. This makes sure the pieces match up. Lastly you place special splicing tape over the join and trim the edges. You need the special tape, since it's designed so the adhesive won't ooze out and get on the tape recorder heads.

If the tape has come loose fro the hub inside the cassette, you need to open the housing and use the splicing tape to fasten the recording tape (actually the plastic leader tape) back in place. Depending on how the cassette was made, there may be screws holding it together, or it may have been glued. Some commercially-made music tapes and bargain-grade cassettes weren't made to be opened. If you do open the tape, don't lose the little metal piece with the felt square that goes across the opening behind the tape. This is the pressure pad that keeps the tape pressed against the head in the recorder. Without it, the tape won't work properly.