That is a good price. It looks like a europiccola chrome w/ a black base. Do you know if it is an 8 cup or 16 cup model? Retail for each seems to be around $700 and $900 respectively. The nice thing about simple levers like the la pavoni is that not a whole lot can be go with them, so purchasing one used is pretty darn safe. As long as the heating element works, the p-stat is in good condition and the seals / o-rings are not ancient you should be good! That is a fair price too. Try to get them to ship it for that price
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Make sure they pack well around the machine. I always line the box with home foam insulation that is 1.5-2" thick, place the machine inside and then jam news paper or something compressible around the machine itself. A this side up note / fragile always help on the box too! Also, request it be shipped and insured through FedEx ground. I have never had a problem with them and they are very reasonably priced. I can not say the same for UPS and USPS who have each graciously given me 2 or 3 ravaged boxed with dented machines inside.
I have never owned a La Pavoni, but I have heard the same complaint that you mentioned above about the base being very light. I am sure that it takes a little getting use to, but I am sure you would find a way to work with it.