Mtb help before i buy?

First if all, how much do you think the sycr (i think 2020-2022) is worth in near new condition and then also non near new condition? Im looking at one now but I dont want to get either ripped off or get stuck with a bike i cant sell later. The reason im sceptical is the that the bike is on ebay for 650 (550 with 100 shipping) but nobody is buying it. Many watching it though. Also, a few parts were switched out. I dont know if they are better or worse. How much of a risk would i be taking buying, especially on ebay. I would also like to know the risks on craigslist, buycycle, pinkbike, marketplace, and marcardi. I will post an album of the pics. https://ibb.co/album/r2k7f4 . im especially worried about the 1st pic with it looking like something is missing on the lever thing. Not sure what im looking at. He also said the frame is med, 17 inch but i cant find a syncr at 17 inches.

My 2nd question is, what difference is there in 17 inch and 18 inch frames? Just standover height and the top frame tube is just lower or does it effect other things like reach and seat tube angle? So if i had 2 bikes, 1 18 and 1 17 with the same seat height from the ground and pedals, would the only difference be more leg clearence if i got off the bike?

3rd question is about dirt jumpers. Ive ridden a mtb med 18 frame and ir feels alittle big. Would a small mtb or dirt jumper be better for me ? And as far as the differences, is it just tire and frame size, or does it change the seat tube angle, reach, etc. If i put a dirt jumper seat up to were a mtb would be would i be reaching for the bars more? Pics i see of djs seats look very low. I also feel kind of hunched over on mtbs but riden one with bmx style bars/head set and i like that better, But still feel a little big.

4th. Do bikes need to be maintenced after buying? If so, how often and is it easy to do myself? What would it cost to pay someone? What parts need the most maintnence? For djs, hardtails, and full suspension bikes?

5th. Is there a eli5 for bike parts so i know what im reading and looking at when looking at a ride?