There are always going to be pitfalls when buying used markers with regards to reliability and general condition of the item your buying but that being said the buying from the used gun market is at an all time low with some great deals out there to be had if your not in a rush to grab the first marker that pops up on the forms.
If your looking at an Etha then they are popping up (occasionally) for between the £240-£270 mark compared to £305.00 brand new,and with them being a relatively new marker onto the market they have a fairly limited upgrade path with regards to performance upgrades if that's your thing.
Used Axes are going for around the £300 mark compared to £425 brand new
And used Invert Mini's are going from £100 to £200 depending on the age and updates they have,compared to £285 new,the good thing about the Axe/Mini is they have a fairly comprehensive upgrade path like boards,bolts etc...
I had a similar issue when i downgraded from a Geo 2 and decided to go for either a brand new Etha or a used Empire Axe which are at a similar price and i decided on an Axe simply due to one popping up at the right time at the right price.
If you have any niggling doubts about buying used markers and are torn between the Etha or a Mini then personally i would go for the the Eclipse Etha,it's a fantastic price for what you get and they are virtually bomb proof with Eclipse's fantastic customer service and you can even stick a milsim body kit on it if that's your thing.