In my personal experience, I've never had a problem worse than tardy shipment; I've always received what I paid for. But, your concern with a new seller with no feedback rating is not unreasonable.
In response to fraud which has occured on E-bay, they have made available two different services to protect buyers. The first is third-party escrow (http://pages.ebay.com/aw/safeharbor-escrow.html), the second is insurance which E-bay provides (http://pages.ebay.com/aw/safeharbor-insurance.html). E-bay is also now requiring new users who register to give a more legitimate e-mail address, rejecting e-mail addresses from those free services.
Use a money order (preferably from a reputable source, like the Post Office) so you will have good documentation of the transaction. This way, the seller has no excuse to delay the shipment while a personal check clears. Send the money order to the seller via USPS Priority Mail so that you are fairly certain of the delivery date. Also keep in communication with the seller, tell him when you send the money order, ask him to tell you when the money order arrives and when your calculator ships. Be polite in all you communications. Give the seller a reasonable time to respond to e-mails and to ship; sellers have lives too, and have things happen which will legitimately delay completion of the transaction.
Trust your intuition; if something doesn' seem right, even if you can't articulate what is wrong, don't bid. There is enough activity on E-bay that you will see another instance of that calculator in the future.
Good luck.