First, I strongly suspect that no variant of the OptiPlex has the 6-pin connector, so, if your video card needs the 6-pin connector (a little surprising on a 750Ti, but I guess some variants of the 750Ti might require the connector), then you're probably out of luck. I could be wrong about the PSUs not having that connector, but I'm FAIRLY sure they don't.
That aside, if your GPU does NOT require the connector, I can say this:
Having used an Inspiron 3647 Small Desktop (Haswell era) with a 55W card and a 54W CPU, on a 220W PSU (216W max on 12V rail) I'd say that it SHOULD work. My son ran this for well over a year gaming with no trouble.
From that document
@PC Tailor linked to, even the worst-case Small Form Factor has 17A on the 12V rail, thus 204W available.
Seems like this machine runs Skylake CPUs, so, as long as you run a non-K CPU, you're looking at a max of 65W for the CPU, and 60W for the 750Ti. That's 135W if running maxed out, leaving 69W for everything else.
That's pushing things really close. If you're running a lower-powered CPU, it gives you a little more headroom.
The desktop variant is 17.8A on the 12V rail, thus giving you slightly more headroom (9.6W extra), for a total of 213.6W on the 12V rail. The mini-tower version gives two rails, with on-paper 312W total, but the max limit for both 12V rails combined is 240W according to the document. That 240W gives you even more headroom.
I'm saying all this stuff with the different variations because I can't see the ebay link at the moment, so I don't know if you're looking at the SFF, Desktop, or Mini-Tower version.
My understanding is that, in the past several years, Dell's PSUs have gotten much better in quality. I've had no trouble with them (3 different XPS Towers with the standard 460W PSUs, and that 3647 with the 220W PSU) even when maybe running a bit more power-hungry of a video card than most people might assume should work.