This is going to be a bit controversial but when you want to replace or upgrade your printers put the old ones in storage or use them as back up printers. One of the elements that makes event photography different to many other types of photography is the ability to print on-site. The printers used to do this are usually good quality dye sublimation roll fed printers and are not cheap. Buying one is usually a considered investment which needs to pay for itself in a timely and reliable manner.
Most photographers will have a good selection of cameras and other kit which is common to both event photographers and general photography. The one thing most photographers do not have is a dye sub printer.
The general cost of entry for someone wanting to add event photography to their photography business has fallen quite significantly over the last 10 years. Very good new and second hand cameras, lights, laptops etc. are readily available and at very good prices when compared with the price of similar equipment even 5 years ago.
So, we have these good quality printers which can produce great images and last for years and then you decide to replace one and sell it to someone who can promptly start competing with you. Your new competition wont have the same experience you have but will now have a printer which has cost between a third and a half of the price of your new one. You have just equipped your competition. For the £400 to £600 you will get for the second hand printer you are helping someone take a piece of your business. Would you sell a good chunk of your business for £400?
You cannot stop someone competing with you but you certainly don't have to help them to do it.
After all the above I sold my printers!!
A change in the business and a change in general direction for myself meant that I no longer needed a small container load of dye-sub printers. For the time being I have kept one printer which will allow me to service the smaller jobs currently on the books. I may just keep it as it is convenient to be able to produce a quick sample print sometimes.
Update to the update:
I now have three printers but no real intention of getting any more any time soon. An opportunity to develop a part of the business in a way I had not seriously considered required a dedicated printer. So, I'm now back to a small pile of printers. A lot smaller pile than I had but still a pile more than I had envisaged not too long ago.