The Dear Rich Staff doesn't think you should worry about it. You're okay reselling pool cleaners bags provided that the bags are authorized (not fakes or knockoffs) and purchased within the U.S. The principle that protects you is known as the exhaustion doctrine. (We talked about it here.) This appears to be the general rule although according to this case, some patent owners can establish post-sale restrictions on patented products if the purchaser agrees to the license. In the case cited, that 'agreement' occurred when the consumer accepted a price incentive to purchase toner cartridges. So if you are about to buy pool cleaner bags and you see language in advertisements referring to a "license" to use the product -- for example, "By opening this package, you agree to the terms of the license agreement ..." then you may be bound by those terms provided that you can see the license agreement before you purchased the bags. In general we think this is unlikely. Although we don't own a pool, or a robotic pool cleaner, we did examine our vacuum cleaner bags and found no license language.
What you can and cannot do. It's also possible, though unlikely, that if the bags you buy infringe a patent, the patent holder will move down the food chain and sue you as well. That's because anyone who makes, uses, or sells an infringing device can be liable. Some things you cannot do: you cannot remove trademarks and resell; and you cannot open pool bag cartons and place additional merchandise inside; you cannot combine other merchandise with pool bags and create the impression that the patent holder or pool bag manufacturer endorses your combo-package. Also, from an ecological POV, you should not fill the bags with mermaid repellent.