I buy preloved bags for less than retail, in the hope that keeping them nicely and treating them well, they'll RETAIN most of what I paid for them. But I never buy bags expecting them to increase in value (ie what you want an investment to do). in my opinion, unless you buy at RRP a rare bag and sell immediately to a reseller who offers you a profit (eg some Birkins / and current moment v limited Chanel, eg the heart bags are going for more than retail), you're unlikely to make any money, in my opinion.
Even if you sell on ebay, you're looking at a significant hit with fees. Less than most resellers, but also less of market and more 'legwork' for you.
My day job is selling preloved gold and silver designer jewellery. Even that, where the price of gold is climbing etc, is not a brilliant investment! And that's something that has more intrinsic value in the material (i.e. gold vs leather) it's made from.
Buy what you LOVE. And if you really love it, and can afford it, buy in the boutique to get the full experience. I want a Chanel bag, and having seen the lovely experiences people have in the boutiques, I'm more likely to buy it in a store and accept the value drop as soon as I step out of the door (even if resellers are selling at retail price, that's not what the person selling to them will get)! I see it a bit like buying a new car vs buying used - I've done (and loved doing) both, but depending on the car, sometimes I'm willing to buy new and take the immediate value hit - other times, I'll let the first owner take that hit and benefit later