Tell me why this idea sucks

I'll assume you're talking about the Northern American market, and I'll ignore competing with actual convenience stores;

1. residential zoning versus commercial (kills it before it even starts)
2. not being able to insure the good for theft, damage, loss
3. possible problems receiving pallets of goods in your driveway or at the street curb from a delivery truck
4. the temptation to liquidate any overstock by having driveway sales
5. neighbour complaints about #3 & #4
6. you can't receive deliveries while also be out delivering, assuming a one-person operation
7. raccoons, mice, rats, squirrels, bears, etc may be always at your doorstep
8. everyone you know will want the friends and family discount
9. using some of the product for your own household, and then not tracking it properly
10. typical garages can't hold many pallets, especially if you can't stack them

Hopefully that gives you a few things to consider.

My first thought is it sounds a lot like the myriad of apps already out there that do deliveries from pretty much any store. Why would I use your service over someone else in the space that's more established?

Also, I'd be a bit hesitant to buy stuff out of someones garage/home when I can get the same stuff from an actual store. I guess you could try and compete on price, but any retailer is already getting steep wholesale discounts, so I'm not sure there's much room to compete and still be profitable enough to make it worth it.

Out of the gate, sounds like it's doing too much. Is it a grocery delivery service that specializes in convenience store items (whatever "convenience store" items actually are vs. actual mega marts) or is it a hyper local (within a neighborhood / community) yard sale / sell your junk type platform?

Doesn't seem any different than using instacart / prime grocery / etc, in terms of the first part of the idea.

The second part isn't very unique either, I can already sell crap to my neighbors via Nextdoor.

The big barrier of entry to both of these ideas is that you have to have a decent network effect for it to work (you need both buyers and sellers, and only having one would spell doom). The competition in bother grocery store delivery stuff as as well selling / auctioning off neighbors, even at the niche hyper local level is going to be pretty tough.

Not impossible to break in, but if you can't get the network effect going (think thousands of users) you won't be able to break through. Tough road to travel IMHO.

I would look into possible health & safety regulations relating to this idea. Since you're not dealing with perishable food a lot of rules may not apply, but there might be some issues with storage/tampering.