Best options in order for A1200...
1. Indivision AGA MK2 - sadly out of production. They add some really useful resolutions that your Amiga can't do natively. Get one if you can but the current 2nd hand price is 250EUR or so.
2. OSSC - the Open Source Scan Converter - best used with a 23-pin to SCART cable and a good HDMI monitor. Not perfect, not all resolutions supported, needs lots of time adjusting for best results.
(you must know about this given the line of consoles in your sig, right?)
3. A monitor that does sync to Amiga video. Result will vary from every single mode supported to only some, and not quite centred or scaled correctly.
4. The 8220 arcade converter - mass produced, cheap, terrible quality but basically works.
I do not know what either eBay auction is. Maybe some enterprising person packaged an 8220 or similar with the right cabling for an Amiga? But you're paying for quality - "Fully tested and professionally finished product (no hacksaw used)"
Maybe someone has actually made a new scan doubler. There are lots of designs around and doing it wouldn't actually be that hard - see
https://www.youtube.com/watch?v=l7rce6IQDWs
You would need a triple output DAC in addition to a bluetooth microcontroller that has an ADC. Probably one of the nrF52840 bluetooth board would do it? Not everyone throws a new design open to the world on GitHub and I think it's pretty reasonable if you have spent months making a thing to try and get some reward for it. You might even want to be doing that full-time instead of your day job.
Some will disagree with the above assessment and say the 8220 is fine, but they are only fine in the sense that a filthy, mushy keyboard covered in cheeto dust will still put characters into a computer.
