to be aware of competitive prices for similar goods from other suppliers. Mention this when discussing your proposed purchase and ask your selected supplier to meet the price.
Don't get your hopes to high though - suppliers have already made sure that their products are as keenly priced as their marketing policy will permit. You can't get blood from a stone, and sometimes it's silly to expect otherwise.
You lot must be living in ungenerous area's!! I always barter when buying locally (as opposed to off the net!
You are going to be paying VAT, plus their profit and a fee for having it built so it is the more expensive route..also with off the shelf products you will be purchasing bits that you don't actually require.
Take a wad of notes into a local shop and buy the bits..alternatively..shop on line for the bits where there are always deals and build it yourself.
Decide first exactly what your requirements are.
NB..I barter whenever I take cash..regardless of what I am purchasing...with the exception of the big retailers. It works if you have the nerve.