Some comments when buying calculators off eBay or other second hand sites. I have seen several times a seller with a lot experience and good rating selling calculators for parts. When I started in this business I bought from them a couple of times. In general:
A different thing is when you buy a non-working calculator from someone in eBay that doesn’t appear to be an expert. I am mostly focused on HP41cl, and I have had my fair share of mistakes, having aprox. 5 calculator bodies that cannot work or are in poor condition - not good enough for my customers. But the inexpert sellers may have a calculator that is easy to fix (when you talk about the HP41C). In any case, it is a fair bet, provided you don’t spend a lot on it, and the cosmetic appearance is good enough. Please remember to get a picture of the battery bay, to see the situation of its contacts!
A final comment: seen from Europe, buying from the US is so expensive when you factor in all the import costs, that it doesn’t make sense anymore. Either you use the standard post, and then it takes ages to come; or you use one of the main courier companies, and then it is extremely expensive. There are many european providers, with extremely cheap transport costs compared with the US.