I think it's perfectly fine to buy and resell, on three conditions:
- describe goods honestly
don't create a monopoly
keep social contract
honesty: all the obvious stuff of no fakes, don't say it's used if it's new, don't say it's manufacturer refurbished if you did it yourself, don't make up some family member died sob story
monopoly: unlikely to apply to an individual, but I mean you didn't buy up the whole supply of a necessary item so that people are forced to buy only from you at elevated price
social contract: if you were given an item for free on the understanding you would use it, then I think if you later sell it you should give all profits to charity/offer to the original owner. Similarly if I bought items from a charity shop and sold at significant profit I would think it reasonable to give a portion of the profits to the charity.
In your case with the gardening equipment you are making the goods available in an additional market, your buyer didn't want/too lazy to buy from another market (the store). You have not misled about the other market, so fine. In addition, sounds like you are providing an additional service by part-delivering.
The barbecue set example is improving the market: effectively the $500 item is now available all year at $450 (the $200 sale price is not available now and nor has that been prevented in the future). So both of those cases are forms of market making.
If you buy items and clean/repair/upgrade them, or offer a warranty on used items that normally sell with no warranty, then you are enhancing the second hand market. Great.
I've done this in a limited way myself: a few months ago I wanted to replace the LCD on one of my laptops (to save money I had deliberately earlier bought the laptop with a broken screen as I use it with external monitor). Screen would have been around £30, it was a case of want rather than need, I thought to myself I can do better than that...so I bought another one of the same laptops with a working screen but dead motherboard. Bought for £45. Took screen, webcam and put in my laptop, kept extra charger as spare. Sold battery, RAM, hard disk, keyboard, DVD drive, Wi-Fi card, couple of covers, in total for £95. Probably spent £10 in fees and ~£10 in postage (all via eBay), so ended with free replacement screen, webcam upgrade and still made a profit of about £30 to cover my time selling the other bits. You do have to allow for the eBay fees and value your time for the bits you don't like (playing with laptops I consider fun, but listing & posting are a chore).