Most forums are to allow people to exchange ideas & opinions on the subject of the forum, in this case crossbows.
If you had come to the forum asking if people thought your device was practical, orif there was anycommercial interest, the response might have been that there probably isn't going to be a huge market for it.

For bows like the Excaliburs there is already a stringing aid offered by the manufacturer. For those who don't have a stringer there is always the inexpensiveoption of a "slave string" which is simply a string long enough to go on the limbs with the limbs fully relaxed (bow unstrung). I use a "slave string" for my Excalibur and it works quite well. Simply put in on the un-strung bow, pull it like you are cocking the bow until it latches under the "claws", andput the safety ON. Thenput the new crossbow string loops through the "slave string" loops and over the limb tips. Slowly de-cock the crossbow like you normally would (release the safety - pull backon the "slave string"forcefully - trip the trigger to release the "claws" - let it down carefully). The new string will "pop" under the "slave string" loopsinto the string grooves and you're done! The "slave string" weighs practically nothing (the weight of a crossbow string) and I can roll it up and carry it in my pocket or pack right with my spare string. If I loose it I'm out less than $1.00 for material and the time it will take me to make a new one (probably 15 minutes). BTW - I do make my own endless loop strings even though I use a Boo flemish twist string on my Excalibur.

Your device looks like it should work, but there is probably very little demand for it in all honesty. And I'm sure the issues of safety and liability would be a concern to perspective buyers.
If I were interested I'd be asking what the loops that go over the limbs are made of and if there was a possibility of damaging the limbs. Are the S hooks tested to withstand the pressure? I've used the cam-lock type buckles you use on load tie-downs on my truck and I know they can sometimes jamb and be very difficult to release, so I would be reluctant to trust them too far.
Sorry if some of ussound negative, but when you come to a forum like this trying to promote sales or createa market for products you are making or interested in being a distributor for (like the Middleton crossbow) you need to be prepared to answer questions and accept negative comment. If you can't take criticism, then you probably should stick with marketing on eBay. It looks like your stringer has received several positive feedbacks there, so it might be the best way to sell it. You might also want to try simply informing forum members if they would like to learn more about the crossbow stringing device you are manufacturing and marketing they should visit your eBay link for more information.
http://cgi.ebay.com/ReStringer-RC-crossbow-stringer-restringer-tool-device_W0QQitemZ250203715681QQihZ015QQcategoryZ525 09QQssPageNameZWDVWQQrdZ1QQcmdZViewItem