I love this service. So much easier than any other way to get BTC.
Suggestion: Have three-tiers of purchasing
Lowest level: Can only purchase a very small number of BTC, using your current structure.
Middle level: Can purchase more once you send them a letter containing a password to their
confirmed paypal address. For the cost of a stamp, you can prove that a paypal account is not stolen. This could even be automated using a mailing service like
l-mail.comHighest level: Can purchase large quantities of bitcoins, which are "shipped" to the confirmed address using a password inside a USPS padded envelope (with additional padding to make it is just large enough to qualify for a tracking number). Shipping with a tracking number qualifies the shipper for seller protection in case of fraud. People sell gold all the time using Paypal, so I would imagine that selling BTC should be possible too. Note that you have to get signature verification on shipments worth more than $250.
The biggest risk is that fraud will occur on the highest-level tier, and PayPal will investigate and decide that you are breaking their user agreement somehow and shut you down.
You might be able to get additional protection by doing more of your communication through paypal (for instance, if the buyer gives you their bitcoin address through paypal, you could then prove that you transferred the bitcoins to the address they provided). However, that would rely on the support folks at PayPal not being lazy and incompetent, which may not be a good bet based on some things I have read.
Since you are building a reputation for honesty, you could probably also offer to buy bitcoins using paypal, making it easier for people to sell them as well.
I was saving this idea for possibly using it myself someday, but I decided to give it away to whoever wants it. Hopefully this information won't push up the value of bitcoins too quickly, since I haven't finished buying the ones I want yet. If anyone benefits from this idea, feel free (but not obligated) to tip me here: 19hMEAaRMbEhfSkeU4GT8mgSuyR4t4M6TH