I agree 100% with El Cid, you really cant allow anyone to get their hands on an EA or indicator due to the decompiling problem. Until Metatrader addresses this problem MT is a non starter for serious developers.
You also have the problem with customers requesting multiple trial periods. If people realy must sell EA's the fairest solution is possibly for the customer to pay for a trial period, and the supplier to discount the cost of the trial from any future purchase.
Personally I cant understand why anyone would ever have the need to sell an EA. Developers often claim that they have insufficient capital to trade, and whilst I can understand that, they could of course trade for others under a power of attorney agreement, and retain control of their intellectual property, and make considerably more money with far less effort.
If you flip the argument around, I cant see why anyone would ever actually BUY an EA ! when they also could allow a trader to manage their funds under a power of attorney agreement and pay only for positive results !.
Buying an EA makes no sense, the purchaser has no understanding of the methodology, so they dont know if it's working or not, they have no idea if the software has been correctly implimented, and they get the hasle of making sure the EA is actually running without actually understanding what the EA is supposed to be doing. You'd have to be as mad as a march hare to even consider purchasing one of these things.