As far as your replacement, do be aware you will not get a real QC gearbox on any new lathe smaller than 12". Many of the 9-11" lathes from Grizzly, PM etc only have a partial gear box, intended to control the power feed speed, not for threading.
PM's 12x28 is the smallest current lathe with a full gearbox for threading. Your 618 also has the ability to do a lot of threads, many more than a lot of larger lathes.
Just getting that out there, if the gear box is your main reason for the change.
I like these lathes so have watched a lot of them on Craigslist and Facebook Market Place and have found they don't seem to follow the general logic of machine tools. Anytime I see them over about $700 they seem to sit for weeks before being relisted at a lower price. Tooling beyond the basics doesn't seem to make a lot of difference, but a lack of basic tooling does lower prices.
Considering what I see people asking for just the legs and milling attachment it seems kind of crazy but that is what I've been seeing the past several years. Still it can't hurt to ask the $1300 you paid for it, just be prepared to come down in price if you want to move it along quickly. If you are willing to ship, Ebay prices on these tends to be higher since shipping isn't too difficult and you reach a much larger audience. Being in the LA area may help you as well, lots of potential buyers and a higher average wage, but I do watch the SF Bay Area in my frequent searches, which I would think has a similar market.
I bought mine in 2021 for $650, dirty but light use and practically new with a lot of tooling including the milling attachment. Mounted on a board, no legs. Even at that price it had been listed about 10 days and he hadn't had much interest shown.
I have a theory for the price cap and that is, as you approach $1000 people start to considering the 9" and 10" lathes which without a gearbox are often available at $800-1200. These are smaller / lighter lathes than a South Bend, Logan or Atlas but they are just big / heavy enough that you really don't want to move them once set up, which places them into a weird category between small lathes like Sherline, Taig and the 7" imports and the larger (but still relatively small) 9-10" bench lathes. I also think cheap mini lathes and the fact that Sears sold a ton of these 6" lathes also holds the prices down.
Can't hurt to ask for more, just be prepared to be very patient waiting for the perfect buyer or be willing to accept a significantly lower price. You also may want to consider listing the legs and milling attachment separately with a package deal for a buyer who wants it all.
