There's a lot of misunderstanding about patents. I have 2, and know a little.
1. A patent will not stop someone from duplicating your invention. It only gives you legal standing if you want to take action against them.
2. An invention can be SLIGHTLY different in function, form or shape and not infringe upon another patent.
3. The patent process is VERY expensive, and a lot of the "lnventors! Let us help you!" are just scams.
I suggest if you want to sell these, make and sell them. Should anyone want to sue you, they must notify you in advance they feel you have infringed, and at that point you can either "cease and desist" or choose to fight. For the $8-10k your patent will cost, you would have to sell a BUNCH of fence pullers just to break even. Just my opinion...... good idea, thanks for sharing. I will make one tonite!