You need to look up the particular model. New E61 machines range from $1000 to $3500; and used prices reflect new ones.

As Steve pointed out, the $1000 ones will have a single boiler that needs to switched be between staem and brew. Those in the $1500 to $2000 range are HX machines wich can steam and brew at the same time, but which can overheat. The most expensive ones feature separate brew and steam boilerrs, and often rotary pumps, and are much easier to use. Before buying any machine, you should have an idea which featrues are importatn to you and which are not.

Expobar made great machines but finish was a bit less so you paid a lot less then a comparable ECM or Vibiemme (except the Vibiemme SB which isn't all that good so cheap). Rocket E61s are also pretty expensive but they do perform great which I prefer over ECM and Vibiemme. I always liked Expobars price/performance ratio. You also pay a lot extra for brands like ECM and Vibiemme for a better finish but not necessarily a better performing E61. This is comparing HX with HX and DB with DB.

Besides that you have all the differences in single boiler, double boiler, HX, pressostat versus PID etc like the ones mentioned already here.

For a first E61 you're probably good with a somewhat less expensive second hand HX. They do a great job and can be tamed easily, especially the Rocket models. You always need to know what the owner already did for maintenance as not descaling for 5 years and suddenly using it for a week can cost you a lot of money. Also dual boiler machines have a lot more electronics which can all break down. Maintenance is so important with espresso machines.

Boilers, frame parts, and the like are not universal, but generally are not in need of replacement. I'd avoid anything that has a controller or display problem you can't live with. They are expensive to replace or repair. Valves, gauges, steam wands can often be substituted with a different one should they fail in a way that can't be fixed with common repair parts.

Some machines have a history of problems that are worth knowing about. A common one is running the heating element off a miniature pressurestat without a relay. While maybe ok on 230 V, they can fail regularly on 120 V.

E61 machines around or before the dual boiler, PID, and later flow control evolution are predominantly pressurestat controlled single boiler heat exchangers. You paid a bit more for Sirai pressurestats and plumb in rotary but all the prosumers then (and mostly now) with the standard big brass lever and spring commercial E61s are using a standard part that's been around for a long time and that is said to keep costs down.

When material costs like copper went up the prices climbed past just dollar devaluation so more recent dual boiler PID machine prices are higher compared to single boiler HXs. Depending on when your machine was purchased I think if you base your selling price on your purchase price and its current condition a used older E61 pressurestat HX would go for less than a newer E61 dual boiler PID even if the older E61 HX needed more of a rebuild to make it reliably functional.

Knowing what I know now if I were getting into serious semi automatic espresso extraction I'd favour a used working E61 HX with a rotary pump for plumb in potential. Pair it with an old commercial grinder at a good price like a Super Jolly and there's a set up you can grow into. Use the savings from new for good burrs, parts replacement and mods like E61 flow control on the machine.