You're doing okay if you can find local sellers who actually base their price on value averaging websites like VGPC, or GVN. I've encountered a few sellers who distrust those sites as being somehow skewing prices too cheap. The other week I was picking a game from a local seller, VGPC said $13.50, Game Value said $16, but he insisted on just basing a price from sold eBay listings. The majority of sold ones were in the $9 - $14 range with a few odd ones around $20 to $27.... what did he charge me? $20. Sellers will charge what they want, doesn't matter what the average actually is. They see the higher price, they want the higher price.
My best advice is to just pursue aftermarket items which can facilitate the playing of any and all of the games, on your original console, for a fraction of the cost. These are after all, just shelf collectibles at this point. I paid $20 for a shelf item. Roughly 10 more of those and I could have myself an everdrive with all SNES games at my disposal.
You really have to ask yourself how important it is to have a shelf full of these items, versus just playing those games, because there is almost no winning these days with physical collecting. Everyone knows what they have now, and if they don't, they pull up eBay and you are screwed. You might as well surf eBay, because they're gonna stick that to you.
My only other tip would be focus on collecting current, or last gen stuff. It's always underappreciated and undervalued until it gets to be a few generations old. Right now, nobody cares about Nintendo Wii, but in a few years it's going to be all the rage and everything will be pricey. Look into Nintendo Switch games, they're fairly inexpensive to buy even brand new copies. There's so many interesting smaller print games you can pick up that nobody is currently paying any attention to. In a few years, many will become sought after. It's easy to find vendors selling overstock and closeout of many last gen titles on PS4 and Xbox in brand new condition. IMO that's always the best option, even if used would be a little cheaper. It's better to have something nice and mint by giving an extra $8 or $15 will the option still exists.