I have a few effects that I have adjusted, refaced and made new props to fit in with my act. These are not new "tricks" but are different props for them. I think that there would be some interest in the magic world but I don't want to get slammed because I didn't "invent" the effect.

I would not be marketing the whole trick but just the add on props. Does anyone have any wise words of advice as to how to do this?

I would think an add on would infringe, if it depended entirely on the original device.
On the other hand ,if it also performed an add on function and an additional function that could work independantly of the original and in fact did not need the original,but could be used together, in my mind at least, this would be ok to market.
(like a TT streamer.Could be used its own and doesn't need a TT. But adds on to and enhances a TT)

Anyone else?

A true "add on" would be considered a wholly original work. Adding a gimmick that is not part of the original method, a booklet containing alternate methods, or plans for the user to modify the original equipment for a different effect should not be a problem if (1) your product contains no pieces from the original, (2) clearly states that your product requires the purchase of the other package in order to be workable, (3) cannot be reverse engineered to provide the customer with another person's product, and (4) does not tip the buyer to the other product's methods. If you can safely confirm that what you plan to sell meets these criteria, you may consider playing it safe by writing the originator of the effect and asking his blessings (along with a gift copy of your new product).

If it doesn't fit the above criteria, it's a derivative product. At that point, contacting the original creator is not a courtesy--it's a necessity--if you plan to release it for sale.

I guess the main point of my post is...it doesn't hurt to ask permission, even if you're legally within your rights to release the product without doing so.