Hi. What He says about “good condition”? How many years of usage, what type of water was used, how many shots per day. Descaling of the boiler can be expensive and/or very tiresome. Ask about warranty. Check the heat up time before ready. If it’s very long do not buy, the boiler can be in bad shape. If You are really happy whit the setup, You shouldn’t consider a change. Boiler is a boiler, E61 grouphead is a E61 grouphead. You wont feel a big difference in taste, only in the workflow. Going from eg. Barista Express to Mara X is a big jump in everything. But differences between E61 to E61 machines do not play a larger role in the image. There are HX models and Double Boilers, even few Single Boiler models, but in most cases the operation is nearly the same (excluding cooling flushes or waiting for steam to froth milk). I understand upgrading from HX to DB. This can be a good machine, but are You willing to pay money for a used model, just be aware it can be a waste of money. Especially if You are buying for a business. But if You have the technical skills and want to do some tinkering, You might buy a new boiler from ECM and assemble it my Yourself.