About 10 years ago I worked for a startup that used a sales agency. Unfortunately it did not go well, in the nearly 3 years I was there they never made a single sale.
They didn't understand our product and had no desire to understand our product. I advocated to end the contract with them, but management refused.
So I think I would want some verbiage in the contract that says I can just walk away if I don't think it is working out. I understand you need to protect your interest too, but I wouldn't want to get strung along by someone who is just not willing to put in the work.
Would OP’s proposed performance-based contract, where they’re only paid when the sale goes through, align their interests with yours better than the status quo model you describe here?
No, because of the second-order dynamics:
OP will have multiple clients. OP’s sales employees will get to choose which things to sell.
OP’s customer still has to do a lot of work to bring OP’s salespeople up to speed at all, and even more if they want to be successful/“compete” with OP’s other customers for OP’s salespeoples’ attention/effort.
It can be a good business model for OP if he can get it, but as has been mentioned elsewhere seasoned potential customers will not, and should not generally, bite.
The downside for OP is once the sales process starts working, his customer grows, probably hires in-house sales staff, trains them off OP’s work, and cuts his contract, so he is always having to work to bring in new.