It’s all about friction reduction and rocker stability at higher RPM. Quality and make are important plus material, trunion diameter, ratio (effect's valve lift) and will the rockers clear the valve spring retainer. Plus don’t overlook sometimes when changing to a roller rocker push rod length will be incorrect for the proper contact pattern across the valve stem tip. Purchasing a used cam for a person with an educated eye is no big deal, but to most others it’s a big no, no. A roller cam isn’t critical for lifter to lobe sequence, but on a flat tappet it is, every lifter has to be marked to what lobe it came off of. If you don’t know, shit can all the lifters and buy new ones with the proper break in lube if the camshaft itself passes a good visual inspection. With the addition of roller rockers you will never feel the difference in performance, but perhaps you will with the cam swap, but on the other hand if the cam is not a well match for your set up it can hurt performance. You really have to know where you are now with your performance and present spec’s, remember, one or two or seven parts are not responsible for power production, it’s the entire engine combination. Choose wisely, good luck.